When I was cycling up until about four years ago, I could still wear shorts and T shirt down as low as 17C, nowadays I'm in a jumper with the heating between 19C and 20C.
It's never cheaper to leave the heating on 24/7, but the cost doesn't go up in proportion to the increase in heating time. So, for example if you switch on for 24 hours instead of 16, you may expect to pay half as much again, but in practice the cost increase won't be as much as that.
The economics depend on how you react with the thermostat too. When the heating comes on it takes a long while to warm up, but if it stays on there is no warm up period. That means if you just put the heating on constant without readjusting the stats your average temperature will actually go up. Alternatively you can compensate for this increase in temperature by reducing the stats until the average temperature returns back to where it had been.
Obviously the costings will depend on which you choose. In my case, if I leave the heating on overnight that amounts to a 41% increase in hours. Leaving the stats as they are, it will cost an extra 18% on the bill, but if I reduce the stats to bring the mean temperature back down the cost penalty comes down to about 5%.
I leave my heating on if the temperature goes below freezing, otherwise it goes off.
Measuring air temperature can be misleading, as about two thirds of your body's heat loss is by radiation not convection. Since the walls have much more thermal inertia than the air your perception of the temperature can change a lot more slowly than the temperature measured with a thermometer. For this reason radiant heating is more efficient than convectors. (Think about standing in front of a bonfire on a cold Guy Fawkes night, you're as warm as toast even though the air you're surrounded by is freezing. A thermometer here wouldn't tell you anything very useful about how warm you feel.)