In my experience if you are feeding just the right size there isn't much waste left so they hold onto it for a while with my boas I feed once every two weeks or after a deification if it comes with in that third week I generally do not wait any longer than 3 weeks to at least offer the food but my boas are older my BP well they are young eat like champs and I just watch and note their deification because it could be consistent for a month or two then they may hold onto it for a while.

I think I read somewhere average to expect is once every 4-6 weeks it could be shorter or longer.

Unless you start feeling a "hard solid" mass in their lower end (and when I say "hard solid" as in the mass is thick and dry enough inside them that it actually restricts their own muscle movement in that area - then you worry about impaction) I wouldn't worry to much though a soak can never hurt in luke warm water be prepared that this in itself may bring on a poop in what ever you are soaking in.

This fact has led me to keep a soaking tub specifically for this that can be cleaned thoroughly and sterilized. (had a baby bp when I first owned snakes who we soaked and because of her size decide to use a medium size piece of tupperware from the kitchen it got thrown away)