You can specify which increments you want to appear as available to be booked online on your booking tools: e.g. customers can only make bookings starting every 20 minutes:
Steps to setting the time increment:
Go to Booking tools and click on Online booking settings
Scroll down to the Date and time section
Click on the dropdown for Time increments to display in availability
Choose to allow your customers to book with you starting every 15, 20, 30 or 60 minutes
Your selection will be saved and applied automatically.