I started using Adwords around May 2003. Up until last summer they always took the payment out of my business account around the 24-26th of the month. Then it was around the 20-21st. In November they took it out on the 18th. Is this happening to you to?
I get billed when I reach a certain amount spent. This may help you- https://adwords.google.com/support/bin/answer.py?answer=6393
Hmmm, my bills have been declining because I have been cutting back on my Adwords budget. Its performance has been deteriorating rapidly. I have also noticed that many of my competitors have either stopped using Adwords or have cut their bids back substantially.