How long should I wait to take a 500 mg Tylenol if I just took one?

This depends on your age and weight. It also depends on if a doctor told you to take more or less while treating you. If all else fails read the label on the bottle or call a drug store and talk to the what do they call those guys that make prescriptions in the drug stores, back in the prescription department -a what?  A a pharmacist- one who knows phamaceuticals/drugs/medicine and how they are to be used. They will tell you free of charge ,just make a phone call,it's only 25 cents or less if you use your own phone. : ). It also depends on if your taking anything else.