I'm no expert on this, but I know the amount of breast growth that comes from hormone treatment can vary greatly from individual to individual-- just as it does in developing g-girls. The Tanner scale suggests a "normal" period for a girls development to be around 5 years (age 10-15), but I've know plenty of girls who did not fully develop until 18-19. Since hormone-induced breast growth in males supposedly mirrors that of the pubescent girl more or less, you could be talking as much as 8-9 years of breast growth.
I've certainly known of a number of trans ladies who had surgery after they thought their breasts had stopped growing only to find they gained a cup or so in size afterward. If gaining a size or so more than you wished doesn't sound appealing to you might want to wait at least 5 years.