Good advice from Mark, putting up prices is never easy, and choosing how much is another tough one.
I don't think there is a hard and fast rule, you certainly need to put up prices regularly, but if you've only been going a year then no doubt you have a fair few that you have badly under priced
Sometimes you just have to face up to the customer and tell them, 'Sorry, but we have to put up your price by £xxx amount.' And admit to them you have priced it way too low.
You can also tell them that you have stuck with the price for 12 months (man of your word and so on) but that for you to continue cleaning the windows for them the price has to be viable.
But DON'T HAGGLE!!!! If they don't accept the price increase then let them go, if you feel prepared to haggle, then in your head you must believe the size of price increase is unreasonable.
Tell them to their face if they are in when you call, or leave a note on your invoice, I just say on them;
Price increase as of next clean £xxxx
I don't invite them to call me if there is a problem, it's my business and that price increase is fixed in stone.
If they have a problem with it, they will let you know when the cheque arrives in the post. (I don't do collections)
Ian