Click into Settings > Basics.

You can choose between 15 mins, 20 mins, 30 mins and 1 hour. The default selection is 30 minutes.
Keep in mind that this changes the look of your scheduler, for example, if you choose 15 minutes, your scheduler will show 4 booking blocks per hour. Users will be able to book every 15 minutes and be able to book a minimum time of 15 minutes. 

If you want users to be able to book every 15 minutes, but with a minimum booking duration of 30 minutes, you should set the time increment to 15 minutes and then create a booking condition (under the space settings) that allows bookings that are 30 minutes or greater in duration only.

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