How long would you consider too long for your foot to hurt after you stepped on a nail?

See a physician. If you have stepped on a nail, you should see a physician to evaluate the wound, perform any irrigation and/or wound repair if warranted and to make sure that your tetanus vaccine is up to date. They will probably also take x-rays to make sure that no foreign body is retained.
Nail. If your foot turns red or gets pus, it is infected, it has to be cleaned and treated, that was too long. If the foot turns dark with red stripes that lewad up the leg, that is more serious and is cellulitis and needs hospitalization and intravenous antibiotics, if the foot turns black then falls off, its late. F you can't open your mouth or have difficulty breathing you have tetanus, and may die.