It depends on what I'm trying to do. For NaNoWriMo, you are supposed to write 1667 words per day in order to keep up with the deadline. That is pretty hard for me. Of course, then I fall far far far behind and have to catch up. My most productive day (writing-wise, since I certainly didn't get anything else done) was a Sunday that I spent all day writing. I mean all day. I think I wrote, with only small breaks here and there when Sky made me eat, for ten hours that day. I ended up with a daily total of 8,000 words. I'm not sure that I had time to shower.
For July, I had a more modest goal and tried to write about a thousand words a day. That isn't too bad- takes me about an hour and a half to two hours, generally.
Tennessee Williams wrote for two hours just about every day of his life. I'm sure that I'll never have that much discipline, but I've always been rather inspired by the thought.