Water heaters don't stay on all the time. They are just like a heater or air conditioner connected to a thermostat in a home: they only stay on if the temperature gets too far away from the temperature that the user set. That means that if your water heater is very well insulated, it may only turn on a few times a day - especially after intense use by a shower, a dishwasher, or a clothes washing machine.
If you will be gone for a few days, or don't need hot water for a few days or a few weeks, you should be able to turn it off. However, it will take some period of time (probably at least an hour) for the water in it to reach its normal hot temperature once you turn it on again.