Well that can either be quite low OR quite high mate, like you said there's many variables that come into play, 10 lbs of lbm a natural in his first 1-3 years of training could easily hit that number naturally. So a natty that was somewhat in his noob years of lifting and aided with AAS could possibly.... gain I would say 30-40 lbs in a year at first , depending on what he used and how much . For someone like Phil Heath though 10 lbs of LBM is a pretty big deal. So it comes down to how new you are to lifting, how new you are to AAS, and just what you have run in the past or are willing to run.
Here's some food for thought this is what NATURALS are capable of, so you can only imagine with aas. During the first six months of weight training, one might see a muscle gain of 1.5-2 lbs per month; that sweet newbie magic, where you gain muscle at a rapid rate. It's not uncommon to see that muscle gain accompanied by fat loss.
After six months and through the second year, you might see muscle gain of 1 lbs per month. You're able to increase weights linearly in the gym and everything is still pretty awesome.
Things slows down significantly in the third year, to the tune of about 0.5 lbs muscle gain per month.
In the 4-5th year of training, progress is slow. 1 lb of muscle every 4th month.
5-10th year, 1 lb per year.
If people are only gaining 10 lbs of lbm a year and they aren't too past there genetic limit then something is way off and aas probably aren't the ones that need to be focused on, once you put your focus on why the AAS are not working then that person will definitely be growing way more optimally with AAS in the mix. Also by "too past there genetic limit" I'm talking somewhere in the range of 15-20 lbs of lbm over that limit, then the 10 lbs per year will come into play a bit more.