It varies between individuals, when you're younger and still growing it can be faster (also, noob gains) and obviously the more advanced a lifter you are, the slower the gains become due to diminishing returns/reaching your natural limits, but for a fully grown male 9-12 lbs in a year is probably a realistic number for lean mass gains. 2lbs a month is achievable if you have superior genetics, and diet/training is spot on but that's not most people.
Doesn't sound like much, but it's probably better to be conservative and bulk slowly so fat gains are minimised and you don't have to spend as much time cutting back down at the end of the bulk - after all,
a) most people would prefer to look good all year round
b) better to spend 9-10 months bulking without too much fat gain and only have to cut for 2-3 months to get back to your leanest than put on a load of fat on that will take a lot longer to remove (and you won't want to lose that lean mass you built by dieting too heavily).
/armchairinternetexpert