Morrowind style. I want a slow buildup.
I would agree, but that doesn't say much about the length of the story.
In terms of length, though, I found Oblivion's main quest was satisfactory, maybe I'd complain it was too short if it was all there was to the game, I don't really know as I don't rush through it to complete it as quickly as possible and the estimates of others is not much to go on since different players play the game at a different pace, and a questline that might take one player 20 hours could take longer or shorter than that for someone else, but in the end, the main quest is only one part of an Elder Scrolls game. It's a part I wouldn't want to be without, but it's not the most important aspect to me. I don't mind if the main quest is reletively short because I'm sure there will be plenty of other things to do in the game to keep me satisfied, as there was in both Morrowind and Oblivion. I haven't even completed the main quest many times in either of these games, yet I've played them with many characters and not felt that I don't have enough to do.
In any case, asking for 400 hours for the main storyline is simply not reasonable, indeed, I'd be surprised if I spend that long on the entire game with any given character, counting side quests and guild quests, and I somehow can't see Bethesda keeping the story from getting boring with even a quarter of that length.