There is no certain date where hours just reset to 0. The hours from the day 1 month ago will be freed up again today.
I will copy what I've said in another thread, you may find it helpful to read the billing and payment FAQ.
TLDR: A payment lasts for 30 days. During those 30 days your hours allowance is raised by the corresponding amount of hours. After the 30 days are over the payment expires and the allowance is lowered again.
What that means is a few of your payments expired, to raise the allowance again, you have to make another payment or wait until the hours are freed up again.