I would feel comfortable using vacuum sealed rodents that were two years old. I mean, when I vacuum pack my own meat, I've eaten it two years later and it tastes just as fresh as the day it was packaged. So why would mice be any different?
As far as regular ziplocking, are the ziplocks to be long term storage and once the mice are placed in they're left? Or are they to be used for weekly openings for feeding?
If the mice are placed in, the bulk of the air squeezed out, and then zipped up to be left...I wouldn't say past 6 months, if that.
If you're opening a bag of 50 weekly to get one or two out for feeding, I wouldn't say past 3 months before they'll start drying out and freezer burning.
You have to consider that people live in different climates, everyone's freezers are different, and the brand/style of bags used are different in some cases. So your results could be totally different than mine. I know bagels placed in bags in a low-humidity environment such as Montana kept very well. Bagels frozen here in Indiana, develop ice crystals within the bags pretty quickly and thus the bagels get soggy when thawed.
Hopefully that answers your questions. =)