I don't usually time it, but I put them in some warm water and check on them every 10 -15 minutes. If the water gets cold I change it back with warm water. I know they're ready when they have no more cold spots and feel nice and soft like a live rat.
Larger rats I leave in the fridge for a day or so to let them thaw, then I let them sit in some warm water until they're nice a warm.