Setting a task duration with the mouse is too cumbersome. If the duration is not set, the Duration icon is not shown in the top header part of the task details panel. In order to set it, it requires to click first on the Plan icon to open the calendar, then click on the Duration input to set a value.
Then, even if the Enter key is pressed, the calendar popup doesn't close itself: a click outside the calender is required to close it (that's probably a separate bug). That's too many clicks required for such a simple but essential action in my day-planning process. I like to set the duration of each task before deciding when I want to execute them by placing them on my calendar.
To make it more straightforward, I suggest that you always make the Duration icon visibile in the task header, as well as directly in the task mouse-over menu from the task list.