From Rodent Pro's website:
http://www.rodentpro.com/qpage_faqs.asp
"Q: How long will the frozen feeder animals last in the freezer?
A: Depending on how they are stored, most frozen feeder animals can last up to a year. It is a good idea to minimize any exposure to air by ensuring they are in air tight freezer bags. While we recommend not storing hairless feeder animals for more than 3 or 4 months, if properly sealed containers are used and air exposure is kept to a minimum, they can be kept for much longer."