I usually leave mine about 15 minutes soaking, but just try to watch him. If he looks like he's in distress, you can take him out after 15 minutes and then do it again later, if he looks fine you can leave him in there a little longer.

Even if you don't have a herp vet around, you can take them in to a regular vet and tell them you know it has an infection and ask for antibiotics. If they don't know anything about frogs, you might want to ask for Baytril specifically; I know a lot of people use that for their frogs when they have infections. You can probably order that online as well. I don't have any experience with using it personally though, so you should probably to wait for someone else to comment to make sure this would be okay in this situation....