Hm but that does raise an interesting point: How long is "long enough" to determine whether or not a map is better than another map? I can tell you within an hour or two of playing a game whether or not it's going to be good. What makes a map different?
I don't think you can genuinely. I feel like if you only play an hour or two of the game and it isn't disastrously bad I don't really think you can judge it as a good or bad game overall at that point. Levels, progression, story, or whatever could entirely change after those two hours depending on the type of game. Multiplayer games in general take quite a bit longer in general because of a lot of intricacies in the balance that no one understands in 2 hours let alone 10 hours of play. MB2 has always been one of those games on the higher end of the spectrum where it takes longer to understand a lot of those things than your average shooter. Now, you can certainly make a judgement that the "game isn't for me" in a pretty short time period but I really don't believe you can call any game genuinely bad with such a short time period unless it is one of those ez asset flips on the steam store that took 0 care or effort... that is a bit different.
However, I believe determining whether you like gameplay in general is much easier simply because you're using a majority of it the entire time you're playing. You're directly using the gun mechanics, movement mechanics, aiming, abilities, the list goes on constantly when you are in a match. You aren't standing in the same spot of a map getting to know it just as well as the gun in your hand for anywhere near the same amount of time. Certainly, the design of the maps themselves also play a large part in that. There is absolutely no doubt in my mind about how much the level design impacts the enjoyment of that core gameplay loop. You could have an amazing core gameplay loop but the worst level design in existence and the game would be crap. That isn't exactly what I am going to be talking about here though.
Now in much bigger games I focus on the level design more than others for obvious reasons. Not only to learn and grow myself but to see what they did wrong, or to see how people are reacting and changing to certain things. I have found that the way people react to level design and changes to level design is actually one of the slower processes because a lot of the usage and interaction of the map is more in the back of their mind rather than the forefront. Someone is almost always going to notice obvious issues but there are always a lot of subtleties. Companies like Valve go into this greatly in their in game commentaries and other sources about how just changing little aspects of a map without changing the layout, cover, etc, can have drastic differences on how people move or utilize the level. I am going to use something I did on Republic Cruiser as an example of this. The side route on RC was being under utilized in beta sessions. Now how did I fix that? I didn't change the layout at all I will tell you that. I changed the lighting. Just changing that one little thing caused people to slowly shift that direction more but it was far from instant. Took about 3 weeks for that one change to be utilized even though there were some initial results of that change that made it obvious. Beta is a little slower than live though for obvious reasons though is also with a bunch of people who listen to focus testing changes and read changelogs, etc so in other ways it is also faster than live because the players themselves have a duty to read and understand changelogs. The majority of a player base does not do that.
God this is hard saying in any kind of short manner. Anyway, because a lot of things with level design are in the back of someones mind it takes a much longer time for people to develop a full understanding of a level (in EVERY game). It is pretty easy to develop quick feelings about something but to truly understand it and how it works takes a long time, even sometimes months. It takes months for a proper meta to develop around a level where as it can take weeks for a meta to develop around character changes. There are people that to this day even though they have thousands of hours in Dota for example, do not know everything about the map they play on every day and some of those revelations could have changed several games they played to the point of turning them around entirely but because they are so small in the grand scheme of things they go largely unnoticed. This is in games with thousands of players playing a game at any one moment of time, which means people simply experience a vast majority of different types of players requiring them to adjust and update their strategies more often. This isn't the case with a lot of people in MB2. They have been playing so long that they aren't challenged to change their strategies very often anymore which results in players needing more time to adapt to certain changes.
So now if we look at DOTF we have another problem. Most of the things above allow a good majority of players to understand a majority (but not all) of a map within a few months time period but a lot of the more nuanced back of the mind things with a level that could have made or broken a lot of rounds goes unnoticed for even years depending on the person/game/community. There are so many variables that make it hard. With DOTFv2 we have another variable that a lot of games do not have do deal with. That is the previous map and its sub versions being ingrained in the community for years. This means a VERY solidified meta is in place, even for players that haven't played in a long time. They know every nook and cranny, every spot to jump on someone, they know how far away they can be from the obj and still defend it, they know how far they can push forward without over extending, etc. Since people are used to certain spots existing, certain timings existing for so many years, a simple change is going to throw them off way more drastically because of that. In a way it is almost like getting stuck in a rut that a lot of pro players do in many games where the meta has been unchanged for so long that they literally forget something else is viable or how to think outside the box and when someone new comes in and throws a wrench in the works it causes chaos for weeks in those pro scenes.
That same line of thought, in a way, can be used for DOTF as well. DOTF has to fight that other variable that is the old DOTF, that old ingrained set of "rules" that were present to the point of essentially being habit/muscle memory for a large majority of the player base. Then we just threw a wrench into things. So people have to not adjust to a map like they would in a new game, but also throw a lot of their old habits out of the window. Breaking habits getting through those barriers for the average human takes ~90 days. With some people being really quick (21 days being the minimum found so far) and some being much higher and it really depends on the person and how much time they spend. That number of course doesn't take into account positive or negative factors that impact how fast or how slow players to adjust to a map but just let me note that it has never been a fast process. It takes a lot of information, a lot of time, and being challenged, for players to fully adjust to a level. If players aren't being challenged by other players then there isn't much reason for them to learn and grown in a new environment. And this also applies to other areas in every day life as well.
Over the past few weeks I have been sitting and observing a lot more than I usually do, and also playing. I have said to many people in development that I am seeing a lot of the community, even as recently as yesterday, using tactics that I knew before release would not work on the new map anymore. A significant amount of people have adjusted to some things already but not entirely and not a majority of the population.
Anyway this is a thing that is kind of hard to put into words so I hope this at least gets part of the message across. There will always be blatantly wrong problems will a level that should be fixed quickly that everyone can identify pretty quickly. Like the changes I did last week to DOTF but a lot of those changes were also because I predicted possibly needing to do some of those changes based on my own experience and I just needed more information to "hit the button". Bigger things take quite a long time for the majority of a player base to adjust to regardless of whether or not a couple of them have. As a result you aren't just waiting for you yourself to adjust to the map, but others as well and that takes time. There is kind of a back and forth with this as well, where a group of players learn something but it isn't used against them, then others learn it, then they have to adjust their strategy, etc. So to answer you with an actual number I am going to say 4-6 months from the last significant (ie non minor glitch fix) changes made and I feel that is a little generous because sometimes it takes much less complicated map changes with much larger communities even longer to adjust than that but it really depends on the game and the amount of people playing. There are a lot more factors here than many realize. People have to break old habits, make new ones while also adjusting to a lot of players doing the exact same thing.