While I haven't had a lot of experience with the mice yet, most frozen items will tend to stay preserved indefinitely as long as you can protect them against "freezer burn". Essentially, if there is much airflow around around the item, the moisture will get drawn out of it and you end up with it drying out in the freezer, as well as taking on the freezer smell.
I think my tactic on this would be to divide the shipment into smaller lots, and seal those up in something like the Ziploc (r)freezer bags with as little air in them as possible. If you have a vacuum sealer, that would probably be even better. The more undisturbed the individual "sublots" are, the longer they'll last. The more often the storage bags are opened and reclosed, the more moisture that will be lost from them.
One thing to double check, though, before making your bulk order. I looked into the same thing, and while I could have gotten a great price on 100 frozen mice, the shipping was almost twice the cost of the mice. My guess is the price break is only fully realized on much larger lots than I would need, and by the time I would pay shipping and make the effort to divide the shipment up for storage, the cost savings wouldn't be quite as attractive (for me, at least). Other vendors may have more reasonable shipping costs than the one I checked into.