Difference being, we watched in real time as Anakin went from being a Jedi to becoming a Sith. As to Luke's training, there's no telling how long Luke was on Dagobah, or Han flying flying to Bespin without hyperdrive (while Vader leap frogged them and had enough time to set up camp at Bespin, I might add.) You could call it a 20 minutes, or 20 days, but there's no real way to indicate how much time passed during these scenes in ESB.

And I'll add this as well: Yoda didn't think Luke was prepared enough to leave.

"But yeah, he "learns what he needs" in under six months. That always struck me as kind of lame ... even when I was a kid."

Where, pray tell, did you get this "six months" number from?