I think it really depends on the prey size. I left a rat pinkie in an enclosure overnight once and it was rancid come morning, but I've left rat pups in over night and feed them off to others in the morning. I have enough now that I give it 2-3 hours before I try to see if another one will eat it.

It's pretty obvious when they've gone bad, it's a smell you will remember for a while.