From what I've read it can take 1-3 years with most reaching their adult size in 2 years. With the greatest gains being in the first year. Much of their growth rate (how fast) until adulthood is reached is also determined by how much and how often you feed them. Which is why you hear of some folks power feeding their animals. Can't help but think that has to have some kind of health effect on the animal. The rough guideline they give for IJ's is 24 inches, but you can see from mb's IJs it fluctuates from individual to individual.
As far as when do they stop growing....they don't until they go to that great House of Blues in the sky. Reptiles fall into the category of indeterminate growers. Once they reach their "adult" size, it just slows down to a rate that you don't really notice. Although this line of thought has been challenged through several studies. With those studies, basically they are saying they grow for most of their lives until they are old for their species.
Yeah I may have possibly overdone research, but some of this stuff is quite fascinating.
All but two of my Meraukes are 24"+ with my largest at 28".