Google bills you after a certain amount is reached (starts with 50$) so it is very possible that that happens twice a month. As your volume goes up they start billing each 100$, 200$ etc. until you hit 500$. We get billed at least 3 times per month. You can check your creditcard overview with the account tab in your google account, it should match.
I do wish G had a procedure in place (or maybe I just don't know where it is) to email you every time they make a charge to your card.
You pay only for clicks you've actually received, so you are not billed in advance. We bill every 30 days or after a pre-set credit limit -- £50 - £500, determined by history in the programme.The initial credit limit of £50 is incrementally raised each time an account hits its credit limit before 30 days have ended. The credit limit is first raised to £200, then to £350, and then to £500. The amount billed may be slightly in excess of the credit limit if an account accrues clicks very quickly.