Starting with v7.1 or 8.1, when the add-on opens up the New History window, the activity type defaults to Call and a timer is added at the bottom of the window.
The timer starts automatically. You can pause it and restart it. When you click the stop button, it automatically fills in the Duration field.
If you right-click the timer, you will see an Options menu command which opens up the Options window shown below. This window allows you to define how you want the duration to be calculated. If you set the interval to five minutes and select Any interval started is counted and make a 6 minutes call, the duration will be set to 10 minutes. If you select Round up to the closest interval, it will be set to 5 minutes.