Wait until it's big enough for a live small (adult) rat. I just had my first experience with that tonight

Until the snake is fully grown, I'd advise that you use stuff like cardboard boxes (appropriately sized, of course) for hides. It's kind of a fine line as to what is the right size and what's too small (they like to feel the hide on all sides of their body) and BPs grow fast. You'll go broke trying to buy those little fake caves or whatever every time it gets bigger.

FWIW, I "borrowed" a few unused office wastebaskets from work, cut off the bottom 6" of them, cut a door hole, and my 4' bp seems to be quite happy with them.