No set amount, and they pee or defecate as often as they have to. It varies a lot. Young snakes especially can take longer to defecate simply because they're eating
smaller (younger) prey animals (w/ smaller bones & far less fur) so there is very little waste they need to get rid of. Make sure your little guy knows the water bowl
is there...sometimes it helps a newly acquired (& nervous/shy) snake if you gently dip their chin into their water bowl. Some can take a while before they figure out
where to drink, since they don't see very well & rely on their sense of smell:
I have literally watched snakes trying to drink from the OUTSIDE of a water bowl because they can smell the water is there "some place" & haven't yet learned how
to dip their face into it...

They even make drinking (sucking) motions with their mouth, as if getting water from the dry outer side of the bowl.
Bowls are a little tricky, & something they don't encounter in the wild, especially if the bowl is deep compared to the "height" of the snake. When I bred & raised some
snakes, I used (new) glass ashtrays (found very cheaply in a store) as water bowls until they grew. Ashtrays of that size/type don't hold a lot of water, but they're
heavy (don't tip over) much closer to eye level while the snake figures it out. FYI, most snakes will take a drink if you gently pass their head under a dribbling water
faucet or spray their face from a water spray-bottle...it's like rain.