I wouldn't mind 30 hours, but if people are going to just do "main quests"... then that begs the question, why?
The only thing in this universe that would convince me not to play at my own pace and actually explore the game's map and do at least "some" side quests would be the announcement that the Earth will explode within 3 days after November 18th. In such case then sure I'd want to see the ending of at least my first playthrough before I'd die. But, really, there's no reason to go through it the fastest you can, why would BioWare take 3 and a half years to craft and sculpt such a vast game for people to ignore 60% of it.
Anyway, even if doing absolutely everything that can be done in any given playthrough wouldn't take more than 40 hours I really wouldn't mind at all, the original Mass Effect took me about 38 hours to complete if I recall correctly (first playthrough and I did almost everything, like 90% of it was done, searched, quested, collected, explored, etc). I think that a full ME2 playthrough with all the DLCs took me a good 40+ hours. I'm content with similar lengths of completion for most games. I'm pretty sure, however, that DAI is quite bigger and needs more time to complete it, and when I mean "complete" I don't mean doing just the main quests... I mean exploring, finding secrets, doing at least some side quests, etc. I'd guess that DAI is a solid 60+ hours game, it really looks massive. I mean just look at the map for crying out loud, check the latest videos where you see the player interacting with the game's map in the "HQ room" with all the other characters around the table, it's absolutely gigantic.
Additionally, in that 8 to 10 minutes footage video from GamesCom you can hear the guy explaining the video as it plays that a completionist playthrough (perhaps internally at BioWare for testing purposes) clocked at 150 hours. I don't have the link to that video but it's the one where you see the gameplay with the human female Inquisitor during night time with rain and thunders going on, listen carefully and you'll catch that in the video, 150 hours... that's just absolutely mind blowing. If that's true (and I wouldn't see why it wouldn't be) then let's imagine that you don't have a job and you can spare 8 hours of game time every day. And let's say that during those 8 hours of game time you actually progress, you accomplish things, you don't just walk around and stopping by the top of a hill to look at the scenery for 30 minutes. Let's imagine that you're doing a "completionish" playthrough but without idling too much around and doing things. That would mean that you'd need about 18 days worth of 8-hours long daily sessions before seeing the credits rolling on your screen.
Now THAT is what I call an epic journey.