Set default duration for new iCal events – Apple Calendar App
I use my calendar for reminders so duration really doesn’t matter. The calendar looks much better when duration is 15 minutes. Here’s how to change the default from 1 hour to 15 minutes.
Quit iCal if it’s running, open Terminal (in Applications -> Utilities), and enter this command, then press Return.
defaults write com.apple.iCal 'Default duration in minutes for new event' 15
Launch iCal and create a new event, and it should have your specified duration. If, for some reason, you ever want to go back to the one-hour default, the easiest way to do it is to delete the special setting you created. Quit iCal, go back to Terminal, and enter this command:
defaults delete com.apple.iCal 'Default duration in minutes for new event'