"Well Mrs Kraftyacts it's like this ... last time I did your windows I priced it in contemplation of a regular service but that didn't materialise did it?"
(Wait at least five seconds; count them in your head because you will want to rush on and gabble.)
She will say:
"I didn't realise we were having an extension built." (lie)
"I told you we were having an extension built." (lie)
"That's because we had a new extension ..." (true, but read on - and now it's filthy and the builders have left render and waterproofer on the glass)
Whatever she says doesn't matter because you must say ...
"I will need to look at it to see what state the windows are in before I can give you a price."
She will say, "Well last time you charged me £20 so won't it be the same?"
You will say, " Obviously there will be more windows and they will be filthy from all the building work and so I cannot give a price without seeing them, I'm in your area Julember 32nd I'll call by then and give you a price."
If she baulks at the idea then walk away.
If she lets you go round look at it and in your head put a regular value on it - say £20 when it's up together. Look at her and say first clean will be £40 to get it up to scratch and then either £20 monthly or £25 two-monthly, which would you prefer? (Wait for her to answer and do not speak until she does.)
Any messing - walk away.
If it's now a builders clean - walk away or price accordingly taking care to look out for glass scratches before you start.
Hope that helps.