This depends on the temperature of the water from the time you start heating your hot tub and how hot you are expecting the temperature to reach. It will also depend on how powerful your heating element is for the hot tub.
Typically you can expect about 1-2 degrees an hour. Leave the cover on the tub when heating it up. If you leave the cover off, a lot of heat can escape.
For example a 1000 litre Hot Tub with a 1.5kW heater, with summer temperature mains water should take about 24 hours. Bigger spas, different heaters and winter mains water will all have longer or shorter heating times.
For further information call Canadian Spa on +44 (0)1293 824094