Not so sure with mice Cass but with my female rats I go by the size of the litter they nursed. Less than 10 they get 7 to 9 days minimum, 10 to 15 they get at least 12 days, more than 15 they get 15 to 18 days. Just rule of thumb really though as I also go by how that particular female did coming out of her the maternity tank. Some of them handle litter raising better than others. Some need more of a rest between litters than others. I tend to be very conservative on that and if I'm not sure I just give the female a few more days before she goes back into the breeding tank with one of our male rats.