The development time is almost exactly 3 years. But Oblivion had about 60 people working on it for 4 years, Skyrim had 100. Morrowind, if I recall, only had 20, yet they managed so much.
I really wish they would've spent more of that time on factions, quests and writing. Seems like they spent the whole time just building the world, which while it looks great, there's not much in it. Even the caves get repetitive.
If I remember correct, Fallout New Vegas had a horrishort development time, and yet Fallout New Vegas holds the record for the most lines of dialog in an RPG. Skyrim just feels empty to me. I think it'd feel so much more alive if the NPCs were a bit more interesting and dynamic.