i add money on when they double the time between cleans, majority of mine are 3 weekly and one or 2 have gone 6 weekly recently, i add about £2 to £10 houses, and some i have at fortnightly that are £4 that go monthly i stick £1 on, so i suppose thats about 20%, but you know whether you are pushing it or not with your customers? mine have all been okay though, i just put in relation for them and say you are still saving £8 or £4 a month etc, and they understand that they get dirtier, although it isnt that much but shhh