There's a script that will total the duration estimates of the selected content, which will help you get an idea of how much you may have given yourself to do in a given block of time.
http://forums.omnigroup.com/showthread.php?t=9983
One thing to watch out for whether using a script, the forecast view (which has yet to make it to the Mac), or just eyeballing the list is the behavior of repeating items. Unlike iCal, for example, you only see the next instance of a repeating item, so if you're trying to estimate how much work you've got to do this week, and you've got a daily repeating action with an estimate of 3 hours, your estimate is going to be off by a significant amount on Monday, because you'll only see that task on Monday's schedule, but not Tuesday, Wednesday, Thursday, and Friday. Similarly, if you have repeating items that perhaps don't take much time individually, but you have a large number of them, the view to the future will be overly optimistic about how busy you are going to be.