Personal Records at sixteen years old / goals before 18:
300 lb Bench Press - stopped benching after that though, haven't in three months. / 405
260 lb Front Squat - 6/25/11 / 365
95 lb dumbell incline / no clue
165 lb millitary / 225
135 lb snatch / 225
BW x 20 chin ups / BW x 40
It depends a bit on what phase of training you're in, and definitely on your current body state. When I was a newbie and incredibly skinny -- just beginning -- in 3 months I gained over 25 pounds. That's about 2 pounds a week, and a good deal of it was clean. Realistically you can't hope to do more than that without adding additional fat. If you're past your newbie stage, aiming for 1 pound a week would be more realistic.
if you do it right, you can get 5-6 lbs of clean muscle with little fat gain. just be sure to stay a little above maintenance (varies from person to person, usually 10-20%... i have to use the lower end of the spectrum since i gain fat easily).
Too many factors to consider. Genetics, age, training experience, commitment, etc..
"I am strong because I've been weak. I am fearless because I've been afraid. I am wise because I've been foolish. I am confident because I've been uncertain. I have learned and will continue to learn. Be proud. Just never be satisfied. No stop. No quit. EVER." -MG