I came across a very interesting site looking at the social dimensions of online games. You can find a lot of statistics about average game time for leveling and leveling curves for keeping players motivated. Other statistics look at the stability and cohesion of groups on game servers.

BTW an interesting article about fun, learning, and motivation can be found in Chapter 5 of Marc Prensky’s book on educational gaming. (link)
Find the PARC blog at at http://blogs.parc.com/playon/