Interesting that you should ask that question. If kept frozen and vacuum sealed they can keep up to two years. At least that's what they say on the Big Cheese Rodents web site. They ship theirs vacuumed packed. That is also in line for meat storage guidelines on the FoodSaver site. They don't list mice specifically of course, but they do list other meats.
http://www.foodsaver.com/Brand-Promise.html (halfway down the page there's a chart)
I've been using Ziploc vacuum bags with a hand pump, but they have discontinued them. I just bought a FoodSaver and a bunch of one pint vacuum bags for the 150 mice I just bought, 25 of which I'm sharing with my son. BTW, there are other brands of vacuum sealer out there. Not trying to push one brand over another. This is just the one I'm familiar with.
One thing I also do is keep my vacuum packed mice inside gallon Ziploc bags. Sometimes the hand pump bags lose their seal and having them double bagged helps. I currently have some 8 month old frozen mice which are still fine. They look they did the day they arrived.