The hour system works on a 30 day period. If you are a free user, you are able to bot up to a maximum of 200 hours in one single 30 day period. You can view your current hours usage here, by filtering by "Last 30 Days"
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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.
On November 1st, if you haven't bought 3000 hours, you will have -2700 hours remaining. You will have used 3000 in the past 30 days, leaving you with -2700 hours after accounting for the 100 hours that you used 30 days ago, and the 200 free hours.