Typically, with most antibiotics, you should use a back up method of protection for at least 7 days after stopping the antibiotics.

That said there are only a couple of antibiotics that actually interact with todays b/c pills, so the using a back up method is more of a preventative measure now.

It is still recommended though.

Usually it will tell you in your leaflet in the box of b/c pills and the a/b pills.
You could always double check and ring your gyn or regular doc's office as well.