Now that is the million dollar question and there is not a simple answer. The best and shortest answer is know your animal but that is not very helpful. Normally these guys will repeat patterns year after year and the process is easier if you have more than a couple animals. I always have rats and I always thaw only enough for those that I know are eating. If I have an animal refuse for any reason other than it is in shed I will wait at least two weeks before I try that animal again. I will attempt to feed the non- feeding animals first. If they take it great, if not I know I have an animal that will. If you do have to refreeze you can usually get away with it once, sometimes twice. Any more than that you have exploding rats that are just wonderful to clean up. A good indicator that the animal is getting ready to feed again is a change in activity level. This can be hard to tell with boys sometimes because they could be looking for love rather than food. Once you get through the first season the animal will become predictable. Each animal is different so the first year of maturity becomes a guessing game. Wish I had a better answer for you but that is all I've got.