[brianogilvie]

I don't think I'm the Brian you meant, but I'll give it a shot:

The duration option (with the little clock tooltip) is intended for your estimate of how long an action will take. If you use the View Bar, you can filter out actions that will take longer than 5, 15, 30, or 60 minutes.

That way, if you have 20 minutes before you have to do something on your calendar (also known as "hard landscape" in Getting Things Done jargon), you can show only actions that you think you can complete in 15 minutes or less. It's a way of helping you use your time more productively.