I think Google sets a limit of how much you are allowed to spend before they charge you. If you are new to adwords this limit is fairly low and as you continue to pay your bills the bar slowly rises. My account currently gets charged twice a month, but this varies with the amount spent.
Your credit card is charged based on your AdWords billing cycle. When you create and activate your account with a credit card, debit card or direct debit, Google starts running your ads almost immediately. Google then charge you every 30 days unless your advertising costs reach a certain threshold within your 30-day billing cycle. Google will not charge you before 30 days unless you earn £50 worth of clicks or impressions, depending on whether you are running a CPC or CPM campaign. The first time your account reaches £50 within 30 days, Google will charge you. Then, they let you accrue £200 worth of clicks before charging you within the subsequent 30 days. If you reach your £200 limit within 30 days, Google will bill you and raise your credit limit to £350. Once you reach the £350 limit, you will be billed again, and your limit will be raised to £500, the maximum allowed. Remember that the amount billed may be in excess of the limit since ads will continue to run and generate charges between the time the billing cycle is triggered and the time the charge is completed. The activation fee, along with any other advertising costs accrued, is charged to your credit card at the end of your first billing period.
Thanks for the clarification ju2ltd, your explantation is definitely more detailed and accurate than mine
Hey I can't take too much credit I copy and pasted my reply out of Google the Google Adwords Learing Centre.
My account is new and I reached the US$50 limit, but they still have not charged me . How long does it take for them to bill - 24 hours, 48 hours or a week?
In the UK you get billed at £50 I guess its therefore slightly more than $50 in the US I'll have a look at the Learning Centre.
According to the Learning Centre the billing points are $50, $200 and $350. Soif you've reached $50 then you should have been charged. Is tis your first AdWords campaign if not your credit may be upto $200
This is my first Adwords campaign and I had checked the learning centre. But they have not yet billed and I have to set my credit limit accordingly