Knowing when to fold 'em

I'm in the middle of a book right now, happily writing away, getting (on average) 2000 to 2500 words a day, and all is well. And then yesterday I realized it was 4:00 already, and I need to get my son from karate at 5:00, and I still only had about 300 ords. That's a very good time to sit down and analyze what's going on.

Almost every writer in the world will tell you something different about writer's block: some say it doesn't exist, or that it's just an excuse, or that it's a sign of a...