Is it safe to leave a thawed rat in my ball pythons tub? For how long?

I'll leave a f/t for an hour or so, I've never had any of mine take one after that point. It seems that if they're going to eat they usually do by that point (although 2 hours wouldn't really be a big deal). Remember though, it's the breeding season right now and a lot of BPs (especially males) will go off food/get super picky for the next few months. So even if all of your husbandry is good, and it sounds like it is (you're not taking him out of his cage to feed, you have a temperature gradient etc.) sometimes they just don't want to eat. As long as it's fully thawed, and not cooked (never defrost in a microwave) there's not a lot else you can do than leave it with them and wait.

I'm not even *remotely* an expert, but I get the impression that you can leave it in there for maybe a few hours without a problem, but not more than that. Alternately, if it's been in the cage for less than an hour, you can freeze it and thaw it again later, once.

I have left rats in over night without problems. If the rat is not eaten, pull it the next morning. I'd leave it more to the cool side of the tub, though.