Bottom line here is that the more cool the temperature the cooler the stones should be. If the stones can’t be handled by therapist or burn your skin immediately after contact they are too hot. In cold winter months your skin is more chilled and therefore a hot stone will be experienced as much warmer than during summer when the skin are warm to the touch.
If you would like more heat rather opt for double towels over the stones rather than hotter stones. The stones themselves should not remain static for long periods. The increased circulation can often leave the skin itchy. Removing the stone and a light tickle will usually remove the sensation. Good therapists, place the stones strategically to warm deeper and the more stressed muscles, move them frequently and use them in their massages movements.