Measure Mode On/Off
By using this tool, you can precisely measure a length of time in the timeline. To enable measure mode, click the measure button (which has a ruler icon), and then drag in the timeline. As you drag, notice that the area below the pointer is highlighted in yellow and the time measured appears in the toolbar to the right of the button. This value is calculated dynamically as you drag so that you can immediately see how long a particular event is taking. When you release the mouse button, the time value remains visible.
You can repeat the measurement process, but only the most recent measurement is shown. Click the measure button again to turn the measure mode off.