How long would it take ?

How long in time, with AS and really working out, would it take to get your arm from 16.5", up to say 18.5-19"? A trainer is telling my brother 5-6 months. And thats with 2 mild deca/d-bol cycles for 8 weeks each.
This sounds like a bunch of BS to me. There is no way that is possible, is there ?

Anything is possible with a good diet and good training

Well the thinking about arm size is that you need to add 15lbs of body weight per 1 inch gained in arm size. I think in some case's thats BS myself.
Honest it's hard to say. If you have long limbs you might find it hard adding those inches, I know I do. I'm 6'3'' and compared some of my smaller buddies I find it harder putting on size in certain muscle groups than my friends.