I have an abcess in a root treated tooth, how long for the swelling from an oral abcess to go down once antibiotics are being taken?

Drainage is better: If anything, the more important question is how long can you go without draining the tooth abscess. Though one may get away with small pockets of pus on just antibiotics alone, more often than not, in a day the pain from pus-under-pressure gets so unbearable that it needs drainage. Otherwise the pus may leak / erode into your sinuses or even worse, cause a meningitis.

Depends : This is tough to say. Your rate of healing depends on many things such as 1. How extensive the oral abcess was 2. Your general health 3. What bacteria caused the abcess. It is great that you had the root canal accomplished, because now you are on the path to healing. Please call your endodontist for specific answers to your particular situation . Take care..