Ours has been on for a while. We compromise with the house between 19c and 20c (my wife would have warmer), so generally the rule is:
June: Some sporadic heating required
July: Generally no heating required
August: Generally no heating required
September: Sporadic heating required.
Since we returned from holiday on 25th September it has been on most days.
If we set to 17c like many people seem to, it would hardly have been on. But that just seems too cool.