Not very long, I like to check mine with a temp gun and try to get the head pretty close to what a live rat would be at. You don't really have to be that scientific to be honest, that's just what I do. I think what really helps in my case, is I do this in my reptile room. Heating up a feeder for one snake gets the smell drifting around, and as I got around feeding the others start coming out trying to see what's up lol.