Hours of fun!
If you love books of any kind, sitting in a cafe, or unusually large stores, this is the place for you. For being a free stop on a trip, it's definitely worth it. I've been to Powell's multiple times, and have always enjoyed looking through the various rooms of books.
I'll admit, from the outside, Powell's doesn't look like much. In fact, it doesn't even look that big. However, once you're inside, you find out why the rooms are color-coordinated, and a number of maps can be found all around the store.
I would give this store a minimum of 1 hour to look through (unless you really just don't like books...in which case, why go at all?) However, I've easily spent multiple hours here.
The best part is, if not everyone in your group likes books, there are tons of other stores within easy walking distance, as well as restaurants, so this store could also serve as a meet-up point while people are looking inside of it.