I suspect this sort of thing varies so much from group to group that trying to quantify it with a rating would be quite difficult. As an example, I recently ran "None More Black" from Doors to Darkness for my group, believing it would take about two four-hour sessions. But my players like to get into their characters, they plot and scheme, they talk to everyone and chase down every possible lead (sometimes down dead-ends, which led to more NPCs created on the fly, which led to more character interaction, etc etc) ... it took us six sessions, and it could easily have gone another. I'm not knocking the idea of ratings, necessarily, but I do think the qualities they're trying to measure are so dependent on Keeper style and group dynamic that I'm skeptical they'd be of much practical value.