For the snakes that are on paper towels, I replace as they get soiled. If a snake soils it while eating, I give them 2-3 days and then change it. With the way I feed my snakes, they don't poop right after eating, so that hasn't been a problem, but if they did I'd change it immediately and just try my best not to pick them up. Do a quick wipe down then, and then do a full clean a few days later.

With the snakes on bedding I spot clean, and do full bedding changes a couple times a year. Snakes that poop more often/pee ridiculous amounts like the retic and garters, and the rainbows with their higher humidity, get bedding changes slightly more often. I clean when they need to be cleaned, instead of relying on taking them out to feed or some other activity to do the cleaning.