How long does a abcess tooth stay swollen after you start antibiotics?

Varies: It could take up to 24hours depending on the size and longevity of the abscess.The strength of the antibiotic and type is also a factor.See your dentist asap.

48hrs average: Takes 48 hrs before the swelling begins to decrease, and perhaps a week before it goes down. It may never resolve completely without treatment.

Soon: If you take antibiotics as directed (don't stop until all the pills are taken), you should see partial to complete resolution of the swelling within 2-3 days. Know however that if the tooth is truly abscessed (nerve of the tooth infected) it will require root canal treatment or extraction or the pain/swelling will return after course of antibiotic therapy completed. See your dentist for advice.