I personally played Oblivion for more than 350 hours on one playthrough and I haven't finished it completely .
I completely finished Fallout 3 however in about 85 hours .
Note : I didn't use the same choices for fallout 3 because I do believe it's a lot shorter than Oblivion or Morrowind .
EDIT : added a new question ....
I personally think fallout 3 new Vegas to be the shortest game of the series , possibly because it wasn't developed by Bethesda Game Studios ....