Each game has different needs, says Valve's head of marketing, which is why the studio's release schedule is all over the place.
The idea of "Valve Time" - a playful dig at the delays that seemingly plague Valve releases - is so persistent that it has its own page on the community development wiki. But Valve doesn't want people to think that these delays are because the studio doesn't have a plan; rather, they're because it doesn't tend to use the same plan twice.
Speaking to Xbox World 360 magazine, Valve's VP of marketing, Doug Lombardi, acknowledged that the company's release schedule looked chaotic from the outside, with some sequels taking six years to make, and others barely a year, not to mention episodic content that is currently three years overdue and seemingly in no danger of appearing any time soon. Lombardi insisted that Valve wasn't "schizophrenic," however, and instead claimed that the studio tailored its approach to the specific needs of each game.
To be fair to Valve, in the six year gap between the original Half-Life and its sequel, Valve put out Counter-Strike, Day of Defeat and Team Fortress Classic, after absorbing the mod teams that conceived them. Similarly, during the extended wait for Half-Life 2: Episode 3, Valve has put out two Left 4 Dead games, and will release a sequel to Portal. Of course, none of this changes the fact that it's impossible to predict with any accuracy when a Valve game will come out.
As long as there aren't any more delays, Portal 2 comes out for PC, PS3, and Xbox 360 on April 20th. As for Half-Life 2: Episode 3, it looks like it will be done when it's done, and there's not a lot of point in speculating about when that might be.
Source: via CVG