I only leave my live rat in with my snakes at tops 10mins, you can tell almost immediately if they are interested in feeding or not.
However on a side note I have a kind of amusing story (not at the time tho). Last night I went to transfer my live small rat to the feeding tank for my snake, well the damn thing slipped and I tried to grab it but i got away. Yes live rat running in my house thank gosh I got it trapped behind some furniture. So after about an hour or so I finally caught it. Man was I happy to see my snake take down that rat. lol