In the Calendar app on macOS Mojave, is it possible to change the default duration for an event from one hour to twenty minutes?
It doesn't seem to be possible. The closest you can get is to open the Day or the Week view, and add a new event by dragging from the begin time to the end time. However, you'll be limited to a duration which is a multiple of 15 minutes:
(of course, you can freely edit the begin and end time afterwards, but that would work for the one-hour events too)