Well, spot cleaning should be done whenever you see something (like what you described) But, fully cleaning the tank (like taking everything out, baking wood pieces, disinfecting plastic items, changing substrate, etc) I think should be done every few weeks to a month? Don't quote me on it though. I clean my bigger boa's tank more often than the others because she sleeps and defecates underneath the substrate. So, about every three weeks I completely change all her substrate, and since I've got everything out of the tank anyway, I clean all of it. Everyone else I fully clean once a month.

When I spot clean, I always take all the substrate out that was in the vicinity of the poopies or urates. You can generally tell by the color of the substrate. And I clean anything that happened to be nearby (like a wood limb right on top of the pee spot).