It depends on the current diet and regiment at hand, the age, and the timeframe in which to gain it.
I personally, could not gain more than 10lbs of lean mass naturally today...in a year. Stop training for a few years, have a shitty diet, then gain 40lbs...very possible, especially with some muscle memory. If you have the calories and train hard, size after 25yrs old, isn't that hard. Gaining 20lbs at 18 naturally...very tough.