Service lengths are configured per barber/therapist. Within Settings -> My Business, click Manage and then Manage Services. Then, click the pencil (edit) icon against the service and update length on the pop-up form:

The drop-down options are determined by the barber/therapist's timetable spacing:

If their spacing is 15 minutes, for example, the options would be 15, 30, 45 minutes etc.

The timetable spacing is shown against the barber/therapist record - please contact us if it needs changing:

Optionally, when editing a service, you can change the service length shown to the customer during booking:

For example, if the service length is 30 minutes but 10 minutes of that is for cleaning, you can display 20 minutes to the customer, while it gets recorded on the Timetable as 30 minutes.