If a client needs a small change made to their website (e.g. a phone number needs changing, a user profile needs to be updated in a way the site doesn't allow, etc.) I usually will not charge my clients. I see it as a small courtesy to them.
But if the client is sending about 10 of these every month, I need to charge for them, and the question is: How much? If they were all given in one go, it'd be easy: I'd just charge an hourly rate. But when they're given one at a time, randomly, across the month, it's more difficult.
While a job (like updating a phone number) may take 5 minutes, it also disrupts your workflow from other clients, as you have to open the project, dig out the right file, make the change, connect over FTP and upload the file, etc.
If you try and calculate something based on your hourly rate (which seems like a good place to start), what's the smallest unit you charge for? An hour? Half an hour? 15 minutes? 5 minutes?