A male bp will reach sexual maturity before reaching full size, so "adult" is a relative term, butters420 gave a decent weight range for many sexually mature males.
If I recall correctly, males reach sexual maturity at around 500grams, not saying you should breed at that weight but its where it starts to be possible. My guy is just over a year and he weighs around 1000grams maybe a little more. Honestly they all grow differently, some larger than others.