5 squares with my experience. 2-3 may seem superficial, while more than 5 scares average readers away (even me as a reader, depending on the topic). My solution (I'm not trying to get rich from the Partner program, just keeping tabs) is to divide a longer story into separate roughly 5-minute segments.

Non-Medium member reads don't affect the Partner $ totals, but it seems as if longer reads aren't quite the turnoff for non-members (at least for the few non-members who actually read the piece).