Generally the thermostat: twice a day, during morning then all evening. Have thermostat at about 21 usually but the radiators are quite old and aren't very efficient - tend to have fire on in front room too. But the thermostat is in front room so if it's warm in there, you feel it when you go to the loo or something because the heating's clicked off!
Last couple of days it's just been whenever we need it to be honest.
With Scottish Power and not sure how much gas is exactly, as we haven't had a proper bill yet (haven't been here long) but pay £77 per month for both gas and electric with them.