I'm wondering how long does it take for an dental abscess to completely clear up with 500mg antibiotics ?

It will not cure it: Antibiotics will not cure an abcessed tooth. What did your dentist advise when he/she perscribed the antibiotic? You definately need to have this tooth treated, possible root canal or extraction.

Cause-dependent: This is a type of question that answers specific to your particular condition vary depending on the reason ; severity of your abscess . Typically, you are prescribed 7-10 days course of antibiotic treatment. Most antibiotics usually do not kill bacteria, but inhibits them from dividing more or growing, and take 24-48 hours to take effect. Dental abscess--unless the cause is removed--recurs.

It won't : The cause of a dental abscess is the tooth--just like your body reacts to a splinter. Whether the root canal ("nerve") is infected or dead, or the gums are infected because of excessive plaque/calculus ("tartar") accumulation, the abscess is your body's attempt to get rid of the infection source. Antibiotics will alleviate symptoms, but not treat the source (like cold 'medicine'). Get it treated!