How long does it take for an abscess in the mouth to decrease after taking amoxycillin?

Depends: Depends on the cause, location, severity of the lesion, and what bacteria are present. But remember, the antibiotic is not the cure, seeing the dentist is the cure. Be sure you have a diagnosis and that your treatment is being overseen by the dentist.

How Severe a Abscess: An abscess is an accumulation of dead cells that have begun to liquefy, usually due to bacterial infection. Since there is no blood supply in necrotic tissue, the body will try to wall it off & send in white blood cells & antibodies to control the infection & dissolve/resorb the dead material. Amoxicillin is an antibiotic that can help the body but source of the infection needs treatment.

An abscess: Often times should be drained. A true abscess can be walled off so that antibiotics may not be able to reach it so well. If after 5 days there is not significant improvement of being on the antibiotic and whatever gargling instructions you may have been given, i would return to the dentist/ oral surgeon.