I've mentioned before that I like writing at a steady pace of a 1,000 words a day - this includes just pure creative writing, not editing, emailing, blogging, commenting, hanging out on forums, etc. A concentrated effort of about 1K words takes me around one hour to complete (without revisions). It doesn't sound like very much, and indeed there have been times when I churned out 2,000-3,000 words per day.
So why don't I usually write more than 1,000 words a day?
1. Avoid burnout - a 1,000 words a day, weekends off, is 5,000 words a week, 20,000 words a months, and a complete first draft of a 100,000-word novel in 5 months. Another month for revisions/editing, and theoretically you can have a complete novel in 6 months. That's a very good pace. Much better than writing in 3,000-word spurts that, after a week, leave me so wrung out I need a month-long break from my novel.
2. Leave time for real life - I have a husband, three children, a dog and a bunch of chickens, and no maid, nanny, cook or gardener. Need I say more? I need to prioritize very carefully if I want to incorporate writing into my life without neglecting other things.
3. More and faster isn't always better - I often find that the best ideas and most creative plot twists actually come upon me at times when I'm doing something not remotely connected with writing, such as washing the dishes or making Lego constructions with my kids. These lulls in writing are just as important as sitting down to my computer and plowing on.