First, if your buying in bulk or gassing your own break them down an vacuum seal them. The sealers are cheap for good reconditioned ones.
As for how long before freezer burn. Depends on your freezer, packing an the like. If you wet thaw a freezer burnt rodent the smell is lessened. Like taking a badly freezer burnt steak an soaking it in water to thaw. Then cooking on a grill. It won't be the best but it will be eatable.