It'll be done when it's done. God only knows when that will be, though.
Note to Valve: releasing a new expansion pack every 3 years and tacking the word "episode" onto the title does not constitute episodic gaming. Seriously though, Episode 3 had better be ridiculously good and about the same length as a full game, otherwise a lot of people are going to be extremely pissed off. I understand that they want to make it as good as it can be, but it gets to a point where it's just stupid.
Neither of the previous two episodes have been particularly long; hell, they were actually shorter than your average expansion pack. Surely there's only so much polishing you can do? And yes, I do think HL2 Ep3 should be their priority, not L4D2.