How long does it take to heat your hot tub?

Depends on the size, the type of heating element, the water temperature and the ambient air temperature. I used to have a 300 gallon hot tub with a big natural gas heater; it took about 3 hours from "stone cold" to "102 degrees". I now have a 200 gallon tub with a small electrical heater; from 40 degrees to 102 degrees now takes about 20 hours.