After 10 weeks, I usually stop gaining. I know many people who experience the same thing (8 - 12 weeks and they stop growing).
Now, people throw around this term "receptor burnout" and say that your receptors burn-out after a while, and a lot of people say that's BS.
I do know this, however: as with any drug, the more you take, the more you need to take to get the same effect.