Well, if water gets dirty it's changed immediately, if not then it gets changed once a week. Bedding I spot clean every day (I use aspen on top of a layer of newsprint) and change everything when the paper underneath is visibly yellow. This is more often with my corn snakes and less with the BPs: I'd say once a week with the corns and twice a month with the BPs.

And I fluff the aspen about every 2-3 days since they love compacting it. Or tunneling in the case of the corns.