When paying by credit or debit card, you will be charged at the end of the month (Ads Manager may sometimes charge the card on the first day of the following month) or when the total ad spend across all campaigns reaches the billing threshold associated with your ad account. 

Your billing threshold is based on the history of payment activity for your ad account. If your ad account is new, you will be charged more frequently for smaller amounts. As your campaigns continue and you build a history of successful payments, you can expect to see a longer duration between charges to minimize the number of bills you receive. 

Note that when your credit card or debit card is charged has no impact on the delivery or pacing of your ads.

If your ad account is set up on Invoicing, then you will be billed every month for the impressions that served in the previous month. 

Note: If you decrease the lifetime budget of an active campaign or ad set, your final billed amount may exceed your newly decreased budget due to a short delay between actual campaign spend and reported spend.