How long does the penicillin take to kick in if my wisdom tooth hurts?

No good answer: If your wisdom tooth hurts because of a cavity or it's impacted, no amount of penicillin regardless of how long you are on it will resolve the problem. If the wisdom tooth is coming in straight and there is sufficient room for it, the penicillin (plus(warm salt water rinses, and otc pain medication) may help within a couple of days. You should see a dentist to evaluate and advise you properly.

Overnight: Usually over night. If no better after overnight, see dentist. For best results with wisdom teeth, make sure you are evaluated and treated by a board certified oral & maxillofacial surgeon.

Few days: However, it's better to visit your dentist sooner rather than later to find out what's going on and remove your wisdom tooth.