The AdMob payment cycle is monthly. You'll accrue estimated earnings over the course of a month. At the beginning of the following month, your earnings are finalized and posted to your balance on your Payment history. If your balance exceeds the payment threshold and you have no payment holds, you'll be issued a payment on or around the 21st of the month. Learn more about the payment schedule.
If your current balance does not meet the payment threshold, your earnings will roll over to the next month and accrue until they meet the threshold.