Extending appointment time before confirmation.

It is possible to change the start time and date, but I would like to be able to alter the ending time of pending appointments. For example, if my base time is 15 minutes, and I believe a scheduled appointment will take 30, 45, or 60 minutes, or more, I would like to have the ability to change that before I confirm the appointment. Is this possible? Thank you very much for your time.