Had a phone call last night - woman wanted a full gutter clean etc, I remembered doing it before. I explained I was up to my ears in work these days with my regular customers and just don't have the time to do it. She replied "I am a regular customer!!!" 
"you did it for me about three years ago!" 
.............. "by the way I'm out Tuesday and Wednesday next week so don't come then."
In a way the lady was correct as called you back to have it done again regadless of when it was done last time as you are not going to clean/clear her guttering etc every month now are you
I would say the average f/s/g clean is every 2 - 3 years for most and guttering & downspout check & clear can be more frequent depending on a number of factors.
I'm betting you done it really cheap last time too so you don't want to do it again if the truth be known but still there is good money to be made if you price jobs well.