Change default duration for Calendar events on macOS Mojave
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)