Wondering if anyone know how long we can store DTT reagents in freezer?

I believe 1 year.

We have found DTT to be very stable when frozen. As long as you run a proper control to ensure the DTT is working each day of use, it should be fine for many years. It may be wise to establish some stability data (what manufacturers of reagents do) that can be used to support its use over time if you are making it up yourself. This can be done by preparing DTT and storing it frozen in aliquots. Then periodically test it to ensure it denatures a Kell system antigen for example. This data can then be used to support an assigned expiration date.