I play games A LOT! The way my life is i need a laptop so having a desktop is out of the question.
I bought my macbook pro in february 2011, I expect to replace it around the early part of 2013, whenever the first big refresh of that year is. I would say if you are gaming on a laptop then the best you can hope for at good performance levels is 2 years, if you are casual you can maybe stretch it to 3-4 years if you are happy to play on the lowest settings available.
It's not a solution for actual hardcore gamers but for people like me that play 4-5 hours a day and need a laptop a 2 year lifespan seems about right.
Other advice would be DO NOT buy one just now, they are overdue to get a refresh.