Here ya go:
Tutorial - Billing and Payment FAQ
I'll C&P.
"Bot hours don't just "reset" at a given time. Your remaining hours are calculated by subtracting the hours you've used in the last 30 days from the total hours that you bought. Here's an example:
Let's say today is October 1st, and you buy 3000 hours. Let's say that you use 100 hours every day. On October 31st (30 days later), you run out of hours, so you can't bot anymore. On the next day (November 1st), you'll regain the hours that you used 30 days ago (October 1st's hours), meaning that you'll have another 100 hours to bot with. The day after that you'll regain the hours you used on October 2nd, giving you another 100 hours, etc."
Whatever amount of hours you used from 30 days ago is what you have as bot allowance today, not the total 200 months worth that you used over the span of the month of May.