I use to breed and sell rats a couple of years ago. I did a lot of research to find out this very question.
My findings were as follows.
By 6 months, things like parasites were completely dead, more common parasites were dead at 3 months.
By 9 months, the rodents have been slowly breaking down to a noticeable point. Causing issues like, weakened skin tissue (one of the reasons why their skin slips off when feeding, or their guts burst during defrosting and feeding), bad smells and the overall nutrition of the rodent is being killed off.
Once any rodents in my freezers reached 9 months, I would throw them out or give them away to willing people. I made it very clear, they were taking the rodents at there own risk and I wasn't responsible for any problems caused. I also asked for feed back from these rodents.
A lot of the time feedback was, the snakes fed on rodents frozen for 9+ months, had diarrhea. That was the only symptom I was told from people feeding these rodents. But diarrhea is not a sign of a nutritious meal.
I always asked for feedback from my rodents and I fed them to my own pythons, I had never had any problems with rodents that had been frozen for less then 9 months and I never got any negative feedback from customers either.