I reconstituted 40mg reta on Dec 25 and used it until last week, for a total of 11 weeks. I'm sure some of it was degraded, but I didn't notice much of a difference. I would always use new needles and wipe down with IPA.
My new batch, I made up in 2mL BAC and separated into 0.5mL aliquots that were placed in new sterile 1mL vials. The extra three were placed in the -20°C freezer. This is not to be freeze/thawed continuously, but just once when I plan to use it. I think next time I will just split into 1mL (ie. two aliquots) since that would last me ~5 weeks.
I was a protein chemist about ten years back. Peptides are routinely frozen, although preferably snap frozen in liquid Nitrogen, without a problem. A single freezing/thaw will not damage the protein that much, but doing that over and over will.