Usually for my girlfriends case, we have gone at it for a couple of days, and nothing has happend. That is when she forgot 1-2 pills. Your case might be different, but if you have the chance, Id would definitely still use protection just in case. It varies between bodies on chances and there is no real way of measuring that.
First of all you can't just start a new pack because you don't want your period. You can mess up your system for years by throwing off your hormones. Birth control is designed to keep you on a healthy schedule for a reason. Secondly even on birth control there is a chance of pregnancy. More than likely you will not get pregnant over missing one. But You should not be abusing birth control.