[brianogilvie]

Not really. The duration field was added as a way to help people filter out actions that could be done in less than a certain amount of time. If you have 25 minutes before your next appointment, there's little point in contemplating an action that you know will take 45 minutes. On the other hand, if you have three hours ahead of you, it can be nice to select actions that will take more than an hour, so you can knock off something major.

Some people have requested that projects have a duration that is automatically calculated by adding the duration of their actions. The problem with that is that most of us don't specify all the actions we need to complete a project; the "natural planning" method behind OmniFocus emphasizes defining at least the next action for each project, but not necessarily sitting down and defining the entire project. That can be a procrastination technique, after all! Also, people are generally pretty bad at estimating how long a project will take, unless they have a lot of experience. As the saying has it, the first 90% of a project will take 90% of the time, and the last 10% will take the other 90% of the time.