simple solution mate 4 weeks OR 8 weeks or if he wont accept that then 12 weeks but slap a first clean price on it.
just do it every other time you do the other custies around it
I'd go with this answer mostly
But also firstly you sometimes have to train your custards up, let them know rain water is relatively pure! (prove it to them if need be with a T.D.S meter ive done this a few times, soon shuts em up) and there fore does not get windows dirty! only dirt can get things dirty!!!
Secondly you must explain the way you need to run your business and the frequncy arranged with them goes hand in hand with the price! so for axample if they miss a clean the price needs to be 25 to 50% more!
If none of this sinks in give then 2 chances then dump them on the 3rd strike! But let them know face to face that you will have to unfortunatly take them off your regulars list. And if they want to use your service in the future you will be more than happy to quote them a one off price, and require one month notice in advance!
I'll tell you now this will change there mond up about messing you about 8 times out of ten! If none of this works they abviously do not want a regular w.cing service, and you are wasting each other time, drop em and find some real customers!
sorry for the long rant but its a topic close to my heart!
Best regards G.B