Depending on the snake I will either leave it in over night or give them an hour or two before I offer it to someone else. This is with f/t too. I've never had live prey survive more than 10 minutes in a cage before the snake wanted it.

If you have snakes in QT DO NOT offer their refused food to any of your established collection. I will offer refused food from my established collection to one of my pigs in my established collection and refused QT food gets fed to something in QT that will eat it.

You could also try feeding at night. I don't offer mine food until 9-10pm. I get great feeding responses from ~95% of my collection. I've also had a few take a few weeks before they felt comfortable eating.