When you first top-up you will get your free allowance within 24 hours (usually almost straight away). This will also set an anniversary date. As long as you top up before that date next month you will get your free allowance again. And so on each month.
A good place to start is the booklet with the phone/SIM and the Help section on the website. After all you wouldn't want to miss out on all the other free stuff you can get from O2, just because you didn't spend five minutes reading up on how it works!