If you want to allow your customers to choose their duration for their appointments, then go to Zovonto Pro -> Modules -> Customer Defined Duration and enable the module.

* After enabling this module, go to each Service to configure the Customer Defined Duration. In Duration Type option, set the value as Customer Defined to allow the customer to choose the duration.

* Set the Minimum Units and Maximum Units which the customer can choose for their appointment in the respective fields.