I can go through 500 in 6 weeks in the dead of winter,,,,
But since moving here ,,we have always had a baby or young children here,,so the heating can be on all day,,only going off at night,,,
But being carfull I can get 8-9 week,,,,
Of course if you were a young couple out at work all day,,,and only having it on in the evening ,,I think you could double it
Our house is an old cottage,,3 beds and detached,,,a new house , might be even better energy wise