I agree that a retainer fee will be the best solutio.

And yes, five minutes here, and five minutes there adds up over the month. I typically bill when the project is more that a few minutes handling a "One Dee Ten Tee" user question or simple direction like reboot, or click here then there to change a setting.

Customer Satisfaction is key, and when billing for an activity showing support that did not include a charge assures there isn't a thought that you are taking advantage of them - assuming of course that they value your services.