I quote a monthly charge, but if they have it bi-monthly add £3, quarterly add £5.
if a custie had it monthly then says leave it this month in january, the next clean (february) is an extra £3. if after that next clean with the £3 charge they have the clean the following month (march) i refer back to the usual monthly charge,
if they say leave the next two months, i'll charge an extra £5 and so on....
i dont charge extra for the first clean
if they just want a one off, i calculate based on £40 ph instead of the number of windows.
ie. if it takes me 15mins-£10, 30mins-£20
they are not worth much as a one off so be greedy with them, but of of course if you do a good job they may change their mind and stay with you regularly so play it how you see fit.........