Personally, I gave up on Everquest many years ago when I could never guarantee sitting at the PC for more than a few hours (and I was single back then!), and WoW is of course much worse if it takes days... DDO is a godsend for my sporadic ability to commit to play.
The static group who have never seen the quest will take hours at least to run some stuff, especially running no spoilers at level. How long might it take such a group to get through their first Shroud ? Granted this group is on the fringe of users, but they do exist. I joined one for lols, carefully avoiding showing or telling them how the quests they were running were done, and they were learning, but rushing anything usually meant death somewhere - even me rushing ahead, despite knowing what to expect. We had fun, even if for completely different reasons. Our play window was 3 hours - enough to run 1 or 2 reasonable length quests, and revisit a quest for higher favor toward the end if they were pretty sure they could do it based on past experience.
Now if a casual group only gets a 1 or 2 hour window to play say once a week, I can imagine them getting deterred by a quest entrance saying "Duration: Very Long". It would be a waste running things like Crucible, Von 5 (+6), quests you can get lost in or have to figure out puzzles or lead rats through a maze...
Also, having a save/restore mechanism of sorts might provide for the ability to create epic length quests - even if you're soloing, a real dungeon crawl could be spread over several hours played across many days. I believe it has potential if thought through and done properly. So I'm for the idea.
But... it won't be easy to implement. Of that I'm pretty sure.
