Your hours are only valid for one payment period. This means you get a certain amount of hours to use over a 30-day period based on the subscription you chose.
If you do not use all of your hours during your 30-day period, you will not be able to save the rest of the hours for the next period.
You always get new hours on the day you pay for your BookBeat subscription. If you upgrade or buy extra hours, the additional hours are also only valid until the end of the current period.