Ok, lets try to get everyone's questions answered here..
Mice give birth every 21-24 days. Some may be longer, it just depends on the individual females.
Google "mouse care" and it'll bring up everything you need to know. I think people are allergic to self-research.
I'm like Sojkas, I keep mine for a year or more and really don't notice to much of a decline in number, size or health of the newborns. I do like to keep my mice and "ooo and fawn" over the pretty or cute ones, even though they are snake food. So tossing out very productive females or females who make awesome mothers just makes no sense to me. By the time they're 7-8 months around here, they've only lad 2-3 litters! Plus I hate having to raise up mice, its long and tedious and by the time I get them raised up they're looking more appealing as the right size of snake food I'm needing for that night. If you only have a handful of mice, feeding them off at that age every time would be pointless. At least in my honest opinion.
As for killing pinks/fuzzies, carbon dioxide euthanasia is long and pointless. Baby rodents such as mice and rats are extremely smother resistant. They'd have to be! Crowded in a small nest with 10-15 littermates and a nest made out of all things smothering. It takes a long time for them to asphyxiate. As cruel as it sounds, the quickest way is to simply put them in the freezer. It takes 2-3 minutes and they're dead. With no or little hair, they lose body temperture very quickly. Since none of my snakes are fed frozen/thawed, I generally deliver a quick flick to the back of the head to knock them out and then feed them off.
And not to sound rude to whoever was insinuating this, but if you can't bear the thought of killing your own snakes' food or feeding them live pinks/fuzzies, why raise it? I just always thought that was the dumbest theory. That's why they have mass-killed, pre-packaged frozen snake food...so you don't have to deal with it. :shrugs:
And Cornsnakekid, it would be nice to have all of your questions on your single posts instead of having to hunt your posts down in other threads and answer them.
You can see nipples on female mice at all times. And like I said in another post, leave them alone! They'll have babies when they're good and ready. It sounds like you're fussing with them way more than you need to, and most times thats when they'll eat all the babies and your snakes will end up with empty bellies.
They typically look like a golfball with legs a couple of days before they give birth. And a way I can usually tell if they're close is if the male is trying to mate with them in their engorged state. I guess the odor they give off just hours before birth is similar to the estrus odor, so I'll end up having a male hassle the female 5-7 hours before birth. Once she starts though, he leaves her alone so don't remove her even though it looks stressful. Removing her is a lot more stressful believe it or not. She feels safe in her home, in a new home she feels more vulnerable and is more apt to eat the babies.
Picture of two very pregnant mice and one who isn't:
And not all mice fit the typical feeder mouse lifestyle and lifespan. I've got a couple that are working on 2 years and still very productive and make excellent mothers, I always get my fattest babies from them. And I even have a 3 year old male hanging around. He's long past being fertile, but he was an $8 mouse from Petsmart and I just can't bear to feed him off.
The better the diet they're on, the better their lives in general..can't stress that enough. Seed mixes should only be offered as a treat, not a staple diet. They should be fed a well balanced lab block. Even though its boring, it has all of the necessary things they need like vitamins and minerals. Which in seed mixes, sure they're enriched...but they SPRAY it on the seed husks and shells, and its not in the part the rodents eat.
Anyway, thats the long and short of it.