If you where a "professional" at a consulting firm in a major city like San Francisco you'd charge anywhere from $80 to $150 an hour with (most likely) a 4 hour minimum. I've worked at the top end of that spectrum as well as fixing things for friends in exchange for a couple beers or dinner. I guess it depends on what your comfortable charging and your relationship with the person your helping. As an individual without much experience you should be able to collect half that with a good conscience and a happy client as long as you manage to fix or identify the problem and tell the person what the next step to get it fixed is.

-Mike