I think giving each game up to a month is a good idea for most games that take more than a few minutes to play. Defender was a good example of when to keep the lifespan shorter, as most of the time the games would be over in a matter of minutes, the only incentive to play was to get a higher score. Longer games, like platformers (Kid Icarus) etc, need more time to play through to completion, or at least to get a good feel of the game. If people do finish or tire of the game earlier then the 'extra credit' idea is great, to keep the discussion going with out having to create a whole new feature game.
Making it a game a month would also give me, personally, a chance to catch up on games that I cant start right away. Typically, unless I have the featured game sitting at home I probably wont get around to getting it untill at least the end of the first week, which only gives 1 week to play it. If I have a busy week / or weekend I will probably miss out all together. Which has happened to me with Kid Icarus.
Obviously other people have different time commitments / or better time management, but it sounds like the people in charge are struggling to maintain the 2 week turnarounds, which I can understand completely.
If you do tire of a game, and also any 'extra credits' then there are plently of other games out there to entertain you, for this reason I think it is better to keep the time frame uniform so that people know when to check back for the next game.
So in conclusion 1 game a month + 'extra credits' is fine by me, and that is my 2 cents!
Keep up the good work!