Even if you're really cooking on your work, you have to stop sometime. I haven't met anyone who can consistently write all day, or write, say, ten pages in a day. At some point, even when you are on a tear, the writing can go from excellent to moribund in an instant. The worst part is, you often can't tell when the transition takes place. When I was writing Mailman, my fourth novel (and the one I had the most fun writing), I would sometimes exceed my allotment, in the excitement of the moment. When it came time to pare down the monstrous thing I'd made, I could tell precisely where I went off the rails each day--it was as clear as cracks in the sidewalk.
Many writers don't have the luxury of being able to maintain a consistent schedule. But when I'm working on any kind of extended piece, especially a novel, I do my best to set aside the same time each day for writing. I'm a morning person--I do almost all my writing these days between 8am and 11am on the sofa, in my pajamas, with my HP laptop (running Ubuntu Jaunty of course) on a little lapdesk that Rhian had before we met, the underside of which is stuffed with little styrofoam pellets to create a comfortable contour for your legs. But it isn't the hours that form the parameters of my working day--it's the pages. I have to write four pages, period. If it's mostly dialogue, or short paragraphs, I finish quickly. If it's complex or sustained narrative, it takes longer. If I'm still excited after four, I might continue for one more, but I learned my lesson with Mailman. More likely, I run out of steam after two, and have to push myself. But the forced writing is usually better than the overrun writing. This is the Graham Greene method: quantity, not time.
Other writers go by the clock. They will write for a set number of minutes or hours. When Rhian was working on getting her chops back after a long break, she wrote ten minutes a day for a couple of weeks, no more. Eventually she was ready to expand. (I dunno how long she's going these days...she is very secretive about her writing habits). I understand the time-limit method, but my mind doesn't work that way--I have an amount of work I can do in a day, and it doesn't matter how long it takes. Four pages is, I've discovered, the amount of half-decent writing I can do, and I am not eager to mess with the formula.
What's your method, hours or pages? Or are you completely haphazard? And if so, how's that working out for ya?