Although hot tubs use a lot of energy, it is tempting to leave them on so that you can jump in when you like. According to Spas N' Stuff, it takes about half an hour to raise the water temperature by 1 degree Fahrenheit. This means that heating your hot tub to 100 degrees could take all day.
If you don't use your hot tub for days at a time, you might figure that leaving the heater on to maintain the water temperature is more energy-efficient than letting it cool and later heating it back up. However, the rate of heat loss is directly proportional to the temperature difference between the air and the water. According to Energy Ideas Clearinghouse, it always takes more energy to maintain the temperature than to heat the water back up.
Heating your hot tub by even a couple degrees can take over an hour. Therefore, those who like to use their hot tubs at least a few times per week will probably benefit from the convenience of keeping the heater on. However, when you're not using the hot tub very often or you're on vacation, you will save money and energy by turning it off.