when does BC become effective in your cycle?

Hi,
I've been taking Microgynon 30 for about 8 months now, and I've had a sudden concern about when the pill becomes effective. Is it effective as soon as I start taking my pill again after my pill-free week, or do I have to wait for 7 days until it is working? I've been at uni since September so it has not been a concern of mine until now. Thanks!