Figure out a per hour charge that's reasonable. If you do a flat fee, chances are you'll either not get enough because there was more work than you thought, or there won't be much work and the customer will feel they paid too much. If you come up with a per hour fee, the customer knows what they can expect to pay, and can tell you whether you can work until you're done, or just work for an hour or two so they can save some cash.