Answer: at any time.
You can turn on your water heater at any time if you have no hot water left in the tank or if the water is not hot enough. But it is likely that you will have to wait several hours before you can recover it. There is no contraindication to using forced walking on the device, for example if you receive friends or family for several days. However, if you have to use forced walking often, this means that the installation is malfunctioning. Either the storage capacity is far too low to meet your current needs, or the thermostat is incorrectly configured. The only real downside to forced use is the increase in energy consumption that it causes. You too, send us your brico question