If you manage appointments with a specific start time and a specific duration and you need to change the duration, follow these steps:

1) Open the WP Booking Calendar > Settings > Booking Form page

Find the "duration time" field and click on edit button.

2) The appointment time duration option defined in format such as this: "Title@@Time Duration", where "Title" is usually the service name and "Time Duration" is defined in the format HH:MM (HH = hours from 00 to 23, MM = minutes from 00 to 59). For example for duration of 2 hours and 15 minutes, you need to define it as 02:15

Default configuration looks like this:

Service A (15 min)@@00:15
Service B (20 min)@@00:20
Service C (30 min)@@00:30
Service D (1 hour)@@01:00
Service E (2 hours)@@02:00

If you need to provide only 2 Services, with duration 20 minutes and 1 hour 15 minutes, then please configure it like this:

Service Name - A @@00:15
Service Name - B @@01:15

The result will look like this: