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.
For example, if the payment threshold for your account is $100 USD and your current balance reached $100 during January, we will send you a payment at the end of February.
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.