Personally, I’m a fan of the ‘20 minute nap every 20 minutes’ school of thought.

Seriously, though, I’m a writer, and I know of many other writers who swear that one 4-hour block of solid writing time a day gives them optiumum output. More time doesn’t significantly improve their wordcount, and may in fact have a negative impact. Of course, writers are always thinking about what they’re writing, so I suspect part of it might just be that the brain needs 2 or more hours of rumination time for every hour of output.

And like the guy says up there, none of you obviously has any kids. :)