the thing you are looking at is the timer for an immersion heater. There will be a hot water cylinder nearby. What colour and size is it?
Presuming that there are no faults, it will be best to set it to run overnight, every night, to fill the cylinder using half-price offpeak electricity. Some tariffs include an afternoon top-up offpeak period. When the cylinder is fully heated, the thermostat will turn off the element so it stops using electricity.
Only if all the cheap hot water has been used up should you switch on the daytime heating. It will take roughly one minute for each litre of water it heats, at daytime cost of about 35p per hour, and a night-time cost of about half that. A cylinder will usually be at least 100 litres (enough for a bath) so will take a bit over an hour and a half. A bigger cylinder is better as it is less likely to be used up.
The start and end times of the offpeak periods vary somewhat, but can be identified by observation of the meter. The meter's timer may automatically turn on offpeak circuits for storage heaters.
A photo of the cylinder will be informative. It may have an upper and a lower heater.
Your brother should look at the electricity tariff he has signed up to, to check it includes a cheap offpeak rate.