Depends. I'm not fanatical about weighing my adult keepers for most of the year (I do weigh females over the course of breeding, though, just for the data). I do also weigh growing snakes, especially holdbacks, because I'm curious if growth is something I can select for.

I weigh them after they poop. So, on their schedule, not mine. It's probably impossible to get a completely empty weight on a snake that isn't fasting, but I try to minimize just how much digesting food I'm looking at. That number goes in the snake's personal record, in case it ever comes up later.