I have to admit that that's the one thing that bugs me about Empire (except wondering why Chewie was allowed to take Threepio into the cell with him) is the timeframe. It doesn't seem like it would take Han and the others very long to do what they did (even if Vader was so intent on capturing them, I don't know if he'd spend a whole year just sending out TIE bombers around the area), but Luke would have had to have spent more than two days on Dagobah. I have to compromise and say that it was probably several weeks, the big time gap occurring in between "That is why you fail" and "I saw a city in the clouds." That was also about the time the Falcon started its journey to Bespin, and we don't know how long that took. And it was in between those two incidents that Luke changed from questioning Yoda and failing everything to becoming seemingly more mature (even though he dropped Artoo).