If you are using the Premium or Premium Plus plan, your broadcast time will reset on a 24 hour basis. (If you are using the Pro plan, you will have unlimited broadcast time.)
The time at which this will reset is based on your prior broadcasting activity. For example, if you started a broadcast at 9am on Monday, then on Tuesday at 9am your time will reset and you will have your full complement of hours to start broadcasting again.
This is to cater to the global base of Mixlr broadcasters, and also helps broadcasters to stick to a regular broadcast schedule, but we do appreciate that some broadcasters may want to broadcast at different times on different days.
If you want your reset to happen at a different time of day, you will need to wait until the time has been reset. At this point the reset is considered 'floating', and it will remain in this state until you start your next broadcast. This will then determine the time of the next reset.
If your time has run out and you do not want to wait for your time to be reset, you can upgrade to another plan and continue broadcasting immediately.