I do one offs
there are several reasons they may just want a one off, moving out, up for sale, letting place out, end of tenancy etc. but also its worth remembering you are a stranger to them, trust is something earnt over time, they may not want to be tied into a regular agreement some people are just like that.
due to the above reasons some are just the once cleans but often they have led to repeat business for me and I may get called 2 or 3 times a year, you may get recommended or they may have a portfolio of rental properties you just never know, I just ask for a weeks advance notice and price a little higher of my average hourly earnings on regular jobs, work always leads to more work