For how long does contact dermatitis normally last?

Depends. Contact dermatitis usually takes several days to improve, assuming you are no longer being exposed to the trigger. If you continue to be exposed to the cause, the rash can be ongoing.
2-4 weeks. Contact dermatitis won't clear up until the allergic trigger is removed. For example, if the cause if nickel allergy you may have to change jewelry and the front closure on your jeans. Untreated the rash resolves in 2-4 weeks. Oral Prednisone will usually clear the rash in 5-7 days.