What you set the thermostat to will depend on how cold you are prepared to accept your accommodation to be - there is no such thing as too high or low because if you feel cold you need more heat and adjust upward to suit. If your heating is timed to come on only for a few hours at a time then it is possible that at times the temperature will never reach the thermostat's setting and therefore at that point the thermostat becomes completely irrelevant. The combination of the type/standard of accommodation/construction and the design of the heating system will decide how rapidly the heating has the desired effect and how well the heat is retained. There should be no relationship between the thermostat setting and the outside temperature - the whole point of having a heating system is to achieve and maintain an indoor temperature irrespective of the weather outside. The design of the system (assuming this is in the UK) should be able to cope with keeping, say, 23 degrees when it is, say, minus 5 degrees outside. In other European countries the systems need to be able to counteract temperatures as low as minus 20 degrees at times, even more.