At page 38 of the WPATH SOC7, the chart says: Expected Onset 3 - 6 months, Expected Maximum Effect 2 - 3 years.
Ok, I will be at 4 years on July 6, and from time to time I get some action from the girls even now. I have had a friend that had a growth spurt at 5 years and Post, and had to have implants put in at year 3.5 taken out for health reasons -- they were literally killing her back!!
It is quite an experience. Evil cackle!!