This one actually can last for "infinity" as the Story Manager continues to generate quests forever if you are into that.
However, the devs have said the game provides 300+ hours.
After watching a lot of gameplay this week, it is clear that this game has much more content that Oblivion, for which a single playthrough typically lasts at least around 300 hours for me, even without completing most of the questlines or finding most of the locations on the map, let alone exploring all the dungeons.
So I'm now predicting around 300-500 hours, for myself at least or for anyone else playing on a harder difficulty setting who enjoys the exploration side of things.
I watched one guy work his way through one truly massive Skyrim dungeon for about 10-12 hours, and he was not even completely exploring it. There were a lot of parts of that dungeon he completely skipped. Playing that on hard difficulty and actually exploring it would probably take two or three times as long.
It probably took more work to QA test that one dungeon than to QA test call of duty.
And there are 150+ other dungeons! Granted, most are not as big as that one, but they all seem to have a lot of content and none of them felt like any of the other dungeons I saw. They are designed in a way where the art direction tells a sort of story.
There are 350+ primary locations and 200+ secondary locations, for a total of 550.