I've been training consistently for about 8 years now and have made tonnes of mistakes along the way. Mostly it was trying to gain too fast and just getting fat.
I have a naturally small frame and generally weighed about 9 stone my whole adult life - see my early post.
Once I got rid of the fat, I was down to 110 lb (7 stone 12!). Since then, I've gone to the gym more than 1000 times.
I just updated my bodyweight graph, which I've kept since ~2008:
As you can see, it's been a very mixed journey! I let myself get much too fat, hoping that I could build muscle faster that way. I got better as I went along and I think I've got it nailed these last couple of years.
If I take out all the "noise" of the fat periods, and just take my low (lean) points, I get this:
I've gained 20 lb (18%) of my starting (lean) weight in 7 years. 12 lb of that was in the first two years. The last 5 years has been much slower.
If you're heavier / bigger than me, I expect a similar % would be expected - maybe more if you're larger framed / high testosterone.
e.g. a typical 11 stone lean guy might gain 17 lb in the first two years, putting him at about 12.5 stone, then another 10 lb over the next 5 years, ending up at about 13 stone.
Bear in mind also that, if you're like me, you have a typical 9-5 job to hold down, plus the usual stresses / interruptions that take their toll on diet / rest / sleep / training.
I think this represents a great example of what can be realistically achieved naturally. (I have been tempted with steroids, but decided never to take them as I want to see what I can achieve with the cards I've been dealt).
They say "TTIUWP", so I might get some progress pics up soon...