There was an article on CNN last year, “Why most people don’t finish video games”, highlighting a stat from Raptr that only 10% of tracked gamers had completed the final mission of Red Dead Redemption (a game taking upwards of 30 hours to finish). The conclusion for single player games was the future being shorter, campaigns of around 10 hours, with further content in expansion packs.
Update: UnSubject has done some sterling work digging in to the numbers and 10% is at the low end, but the stats seem to confirm the shorter single player campaign, more than half the titles taking on average less than 10 hours.
For an MMO player, of course, 10 hours is “not a bad first session”. 30 hours is “a decent headstart weekend (with 18 hours of queuing and server maintenance)”. 100 hours will do for launch week (I seem to recall Raptr reporting one user with 149 hours logged in the opening week of Cataclysm, though that’s just the game client running as opposed to active play), but there’d better be more than that to warrant a subscription.
I don’t envy a company trying to keep both camps happy…
 OK, obviously it’s a certain subset of MMO players. Or a certain subset of all game players, there are probably 100 hour-per-week Spider Solitaire fiends, engaging in flamewars of eye-popping obscenity with those Freecell noobs between hands.