You can get away with leaving the mouse in for upto 24hrs, but its advisable to place it away from the heat source (this will make it go off quicker), you can tell if the mouse is going off as you will be able to smell it.
If you have recently bought your snake it nedds 7-10 days to settle in before feeding or handling takes place, and something that i do is not to handle the snake/s for 24hrs prior to feeding, AS this can stress your snake.
Another trick you can use is when you leave the food in with the snake, cover the viv with a dark blanket and leave for 24hrs, (i managed to get one of my problem royals eating this way)