Snakes really never stop growing. Their growth rate just slows down the older they get. Does that mean your BP will just grow and grow and grow? No, of course not. But they do continue on a very small scale throughout their lives. That's why they shed - they continue to outgrow their skin.

Having said that, we usually consider our BPs "fully" grown at around 3 yrs or so. And that's somewhat subjective as it does depend on how often you feed and prey size. And also whether or not your BP is a solid eater, lol...