I still don't buy that it would take anything close to six years. Not even the twelve hours plus filmed for LOTR took that long combined, post included. The post-production would be quite a lot quicker now in addition to the filmmakers being able to draw on the LOTR films. It doesn't add up in my ears. Del Toro is probably exaggerating to rationalise his choice, or he's adding in an estimate of the time it would take for the MGM situation to clear up.
Totally unrelated rant: the pedestrian career developments of most directors working today annoy me. What happened to making two or three films a year (like Robert Altman or R.W. Fassbinder)? Those kinds of people probably work in TV now. On the first season of Twin Peaks, they averaged an episode a week in filming. On the second, sometimes more. Lost has also had similar schedules, even with all that location shooting. And these are quality productions we're talking about here. (end of rant)