I did a first clean like that as well as clearing out the gutters so say the frequency of the windows agreed was on 3 monthly clean thereafter. I had a gut feeling she was going to mess me about and only wanted a one-off/yearly if that so said "give us a call in 3 months when you want the windows cleaned again". I received an email last week to ask if I could clean the windows again which happened to work out over 14 months later since they were last cleaned. Some people tell porkies so that’s why you should charge at least double for first cleans.
I've been there (a lot). It doesn't necessarily mean they are bad customers. It's just that their requirements are different to what we would term as regulars. When this happens to me, I just tell them to give me a call when they need me again. Most of them do, and I fit them in around my regular stuff. As long as they pay me, I'm happy.
Many of us do add-ons such as gutter/fascia/conservatory roof cleans on an ad-hoc basis. I see these jobs in the same light.
John