There are alot of depending factors including sox. 1000 gram rubber boots should be good sitting still down to 15 or 20 deg. Some 1000 gram boots are warmer but are constructed diffrently than a normal rubber boot, they use a layering system including reflective mylar type materials like what's used in a space blanket.
Most rubber boots are just one thick layer of inslutation.

A neat trick to keeping your feet warm and confortable on stand is to buy a set of boot blankets and some hand warmer chem packs.
Once in the stand take off your boots and put your feet inside with the hand warmer packs. This is gaurenteed to keep your feet warm. What's neat about this trick is you don't have to spend a fourtune heavy cold weather boots just so your feet won't freeze on the stand.You can wear a good pair of comfortable boots that you can walk a few miles in and won't over heat your feet and won't weigh you down. Jim