There may be no "one" limit on the amount of muscle you can add in a week. I think adding a pound of muscle in a week would rock the cat's ass! But think about it this way... even in the most optimal conditions, you certainly cannot add on more muscle than the protein you take.
Fats and carbs cannot be turned into protein, so the protein you build with is from the protein you eat. That could be one limit.
-Fatty