How long does the facial pain last after having a root canal?

Varies. If your symptoms are prolonged, then you need to go back to get evaluated to see what may be causing the symptoms. It can take root canal treated teeth to take months to heal properly. The key is to see how you feel and get evaluated.
Fast. Should improve within a few days. If pain persists, see treating dentist. If still persists, get an opinion from an endodontist. Not all root canals work.
Depends . If the pain goes on for more than 3 months i'm afraid you have a complication related to the injured alveolar nerve and this can be chronic.
A few days. If the pain lasts longer you should contact the dentist that did the root canal. Especially if the pain is not subsiding at all.