This appeared originally as my answer to a question on Quora, which can be found here: How can I have a chance to act in a TV advertisement?
It all depends. If you go to auditions and some audition goes well, why not? Audition calls happen frequently for various advertisements, and it would be beneficial for you if you kept yourself in the loop by contacting ad agencies and casting agents. They make public announcements through various social networking sites and apps.