It depends on the topic, of course. I have a self-help book for my computer science students that talks about the best way to get a computer science bachelor's it weighs in at 64 pages. It's too small to print, but it really doesn't need to be any bigger.

A lot of non-fiction fits in 50-100 pages or so. Longer than a magazine article. But it's not publishable through a publisher if it's less than 250-300 pages. One of the reasons I probably won't work through a publisher any longer.