1: All about the Preferences
Access the Preferences from the iMovie menu at the top of your screen when the application is running. When you open the Preferences, a new window will appear in the middle of your screen.
2: Tab it, click it
Navigate to the Browser tab at the top of the Preferences dialog window. Now tick the ‘Always show clip duration’ checkbox. Instantly you’ll see the timings of your clips appear on the screen.