Ehm, from what I've read, my crappy evaluation is that, in average:
By 1-1.5 year, you get 60-75 % results.
By 2-3 years, you get 80-95 % results.
By 4-5 years, you get 95-99 %+ results. Some people have reported slow, ongoing change for even decades, though.
I may very well be wrong, but that's definitely the impression I get. Soo~ I guess that in most cases, 3 years is a good time to say that results are fairly done with. Though after just 2 years, it's probably pretty close to the final result already.