For me, it went like this: 1st year 145-162, 2nd year 162-176, 3rd year 176-190, 4th year 190-205 ish. From there (years 5 and now the beginning of 6th) I've been as heavy as 224 and as light as 197 depending on my bf level. I'll be honest and say that, at my heaviest, my waist was 37 inches and it started out at 32. So, basically in gaining 80 lbs., 25 of it was fat. But, I dieted most of that off for a contest and now am about 205 with a much more trim waist than at my heaviest. So, in a little over 5 years, muscle gains average out to about 12 lbs a year or so with a little fat gained and lost here and there.
I'm hoping to put on some quality muscle mass over the next two years, but it does get hard after awhile. Basically, training, diet, and rest have to be more and more dialed in as it becomes harder and harder to make gains. Over time, though it gets harder to make gains, you learn a lot about what works for you.