Bethesda worked on Oblivion from mid-2002 when Morrowind was released until April 2006 when Oblivion was released. That was almost 4 years, and there were still suspicions that Oblivion, which was delayed from its initial late-2005 release window, was a touch rushed, and that it could have used another six months of poilish.
Bethesda started working on Skyrim (can't believe that we can finally say that without it being conjecture!) in late 2008 after Fallout 3 was released, and if they release it on 11/11/11 then they will have worked on if for almost exactly three years from pre-production to release. Given Bethesda's track record with these sorts of games, three years, whilst a long time in general for game development, seems to me to be rather optimistic. If it took them 4 years to make a rather under-polished Oblivion, how in the heck are they going to make the shinier and more epic Skyrim in a mere three? Don't get me wrong, that they want us to be able to play the game in time for next Christmas is fantastic, although I wouldn't be shocked and stunned if they end up having to delay the game until April 2012 or something, much as they did with Oblivion. Thoughts?
http://en.wikipedia.org/wiki/Man-year are more important than callender years. If you multiply the number of people working on a project you can shorten the amount of time each one individually works on the project and still have worked the same amount of time on the project as a whole.
Oblivion was created in 4 years with 60 people. 4x60=240. 240 Man-years went into it.
Skyrim, if as you say, was worked on for 3 years with 90 people. 3x90=270. 270 Man-years went in to the making of Skyrim.
Morrowind is difficult to calculate since it's development was halted several times because of other games (Battlespire, Redguard) or switching engines. But 30 people worked on Morrowind and ignoring that they didn't work fulltime on the project, Morrowind took 6 years. Thus 6x30=180 man-years for Morrowind.
And by the way, some work (planning, outline of story, quest ideas, first concept art and engine overhaul) on Skyrim probably began in 2006 (when the Skyrim trademark was first registered) although probably by only by a skeleton crew of a dozen people. By time more and more people were added to the project (those finished with Fallout 3 and the DLC).
Remember man-hours or man-years for your economics class. With two people instead of one you can paint the same house in half the time.