I've had some well wrapped up in the freezer for 6 months and they look like fresh killed when I defrost them. The ones I buy in bulk small and are all matted and yucky whereas the ones I individually killed and froze look great 6 months on. I thing some will be in the freezer for up to a year because I have some huge old breeder does that are to big (in circumference) to feed to any of my snakes just yet.
A little tip...Don't even THINK about breeding mice til you actually have your snake and even then it's not worth doing unless you have at least 5 snakes to feed.