https://www.google.com/support/adsense/bin/answer.py?answer=9905&ctx=en:search&query=payments&topic=0&type=f said:
When do I get paid?
We'll send your check or EFT payment within approximately 30 days of the end of the month in which your account balance reaches US$100, unless a payment hold exists or unless otherwise agreed to in writing (including electronic mail).
For example, if you earn $40 in January and $70 in February, we'll send payment to you by the end of March.
If your account balance is less than US$100 at the end of the month, we'll roll your earnings over to the following month, until the payment threshold is reached. Balances include the combined earnings of AdSense for content, AdSense for search and referrals. You can review the Life of a Payment diagram below for a more complete picture of the payment cycle.