an "average" gentic limit is imposible to figure as have been mentioned... as is how fast you reach it.
Example:
my buddy- was 5'5-5'6 at about 170lb's and 10% body fat at age 22 with 4 years of good training
Me- 5'7 145-15% body fat at 22, 2 years heavy training
Honestly I will not reach his size in the same amount of time for a number of reasons, all mostly having to do with genetics. His training is slightly better, but I feel my diet is better (except when he dieted down to 10% he doesnt pay much attention)
Personally I will not turn to 'roids until I have 4-5 years of good eating/training (already gone up 30lbs in 2 years) allthough I already see my progress has severly slowed.