How long is an antibiotic effective? my 6y/o daughter is waiting for some dental work

Impossible to say: Antibiotics (abx) have varying effectiveness based upon bacterial spectrum, host ( your child) resistance /immune system, area being treated. Usually abx are given to "symptomatic " relief. Increased pain, swelling, fever, redness would be signs that the abx is not working.

Antibiotic: If she is on a course of antibiotics it should be effective for about 30-45 days after she is finished with the course. It will not cure the source of the infection and the infection will come back with a vengeance and may be resistant so the sooner the treatment can be done the better for all concerned.

The length of time..: ...An antibiotic is effective is dependent on several things: where you put the antibiotic (putting it on skin vs. Injecting it into a muscle), the half life of the antibiotic (the amount of time it takes your body to process away half of it...So if you have 1 gram in you and half life is a day, you have 1/2 gram 24 four hours later), and whether the germ is susceptible to the antibiotic at all.

While you take them: Antibiotics for dental problems are typically effective during the time you take them, but are not effective within a day or two once they are stopped. If your daughter's problem is getting worse, call your office right away, sometimes some other sort of emergency care is necessary. Otherwise call for a refill and have her stay on the antibiotic.