To receive a payment, you must hit $50 in net revenue (revenue after Fullscreen's cut). If you earn less than $50 in a given month, it will roll into the next month's revenue until you hit the threshold.
Once the $50 threshold has been hit, you'll receive a payment at the end of the following month.
Example: Let's say you earned $72 in revenue in February. At the end of March, usually on the 30th, you will receive a payment of $72. Please keep in mind that Fullscreen is based in California (US Pacific time), so you may not see your payment until the following day, depending on the time zone in which you live.