In my experience, this is true whether I've been working on my own project or at a company. There may be people out there who can write code for more than 8 hours straight per day but it's impossible for me. Realistically, if I get 5 good hours coding out of an 8 hour day I think I've done well.
I think there's a big difference between working for ABC Company and building your own. When you're building your own, the concept of increasing effectiveness by reducing hours is counterproductive and limiting. Anyone who's ever built anything significant will readily tell you that crunch mode is essential.
