We have ours set to 21 or 22c in the Lounge.
My mother still hasn't grasped the fact that it comes on automatically, and that the thermostat is actually reasonably accurate. I've currently got a hulking great thermometer intended for a green house next to the thermostat to show that it is in fact warm enough for the stat to not kick in yet (I know I should be more sympathetic, but we can be walking around in shorts and t-shirts sweating, and I still get complaints that "it's cold, is the thermostat working?")
It's set to come on at about 6:30am, then be on call all day through to about 9pm as my parents are at home most of the day, and I'm usually around the house at odd times.
In reality unless people are going in and out a lot, or don't bother closing the door whilst they "pop to the shed", or have a smoke by the back door (a pet peeve of mine), it's really only on for short periods of time, unless it's really cold outside.
Having the double glazing, wall insulation and loft done a few years back made a huge difference in how often/long the boiler actually kicks in for.