I would either leave the mouse in until the next morning and chuck it, or get it out and freeze it after a couple hours. You might be able to go up to 4 hours before putting it the freezer, but if it starts to smell I would toss it. By the way, this is based on nothing but my experience with the bps I've had, and I usually just write off the prey item or give it to one of my "garbage trucks ", rather than mess with freezing it.