How long to use back up birth control after antibiotic?

I've been on birth control for over 8 months and recently just had to get on antibiotics (penicillin vk). I continued taking my birth control while on the antibiotics and used a back up method as well. I have had one full week (7 pills) of the 'sugar' pill and finishing my first full regular week this week while being off of the antibiotics. How much longer should I use a back up method of birth control or does it not matter? Thanks!