I always kinda based that on water equiv. Basically 10” of 10:1 snow will eventually glaciate its way down to the equivalent of 3” of 3:1 sleet. Both would have the same 1.00” water equiv. It’s not a perfect relationship, but that’s always been my rough estimate.
So 15” of snow should have more staying power than 3” sleet. There are some caveats though as fresh fluff has more exposed surface area so it can sublimate more quickly than solid ice can.