In my younger days I used to spend 4-6 hours a night leveling up my MMO characters. Back then it was never a huge issue, but now I've got responsibilities and other commitments that consume large amounts of my time. It's very difficult to fit more than 1-2 hours of game play into a single sitting. For some games this is not a problem, but for others it's near impossible to actually get anywhere in the game. World of Warcraft is a prime example of an MMO that requires large amount of time in a single sitting in order to participate in certain elements of the game. If you want to do a raid with your guild, you can kiss the next 3 hours of your time goodbye. If you want to leave the raid early, you are often severely frowned upon by your team members. I used to think this was perfectly acceptable, but now I think it's just bad game design. MMOs should not force the player into large time commitments, and they certainly shouldn't punish them or their team mates for needing to leave early.
When I first started playing Left 4 Dead I was amazed at how easily the game dealt with players joining and leaving. A L4D campaign is around an hour long, and requires incredibly intense concentration in order to progress successfully. So what happens when the phone rings or your partner calls out they need a hand with the cooking? Easy, you just go AFK. The computer automatically detects your absence and the AI takes control of your character. You aren't kicked from the game, and your team mates don't suffer for your temporary loss. You can return to the game at any time and continue playing the campaign. If only such a technology existed in party oriented MMORPGs, so many problems would be solved...
If you asked me what I thought the maximum amount of time that an MMO should enforce in a single sitting, I would say one hour. If MMO designers used this number as their goal when building raids/dungeons/instances, I think we would all be better off. There should also be an automatic mechanism which compensates for the loss of a player, and prevents their team from suffering any consequences.
I should point out that this issue must not be confused with grinding. Time required in a single sitting and the amount grinding required to achieve an element of the game are two separate issues. I'll tackle the grind issue another day ;P