Margo Lanagan on writing. (Via Theodora Goss.) My daily quota is usually only about half the amount Margo mentions (I often set it at 1500 words, which I find comfortably easy i.e. sustainable over a period of weeks, and I will set it lower - 1000 - if I am feeling any strain), but I second what she says, that there's no point going over one's daily quota because it only involves getting a sloppy version that is essentially subtracted from what one can produce the next day.
I should add that I have always been a huge skeptic about the notion that one should write every day: it simply doesn't seem to take into account life's complexities, desirable and otherwise. I remember Lee Child saying at a publicity event for one of the Jack Reacher novels that he'd written it in 81 days of writing, not consecutive but nearly so (it is not always practical to write on, say, Thanksgiving). He writes one of those books each year, not more; Iain Banks, too, writes a draft of a new novel in about three months. A recharging spell is then desirable.
I write something every day (blogs, emails, lectures, letters of recommendation, reviews, interviews, etc.), and obviously I'm constantly reading and would experience a day without reading as one of mighty deprivation, but I am by no means constantly working actively on a new piece of writing. The rhythm of the school year suits me pretty well, only it would be better if I would work slightly less hard during the semester and accordingly need less time to bounce back after I'm done teaching in December or May. I think that a year isn't a good year for me if I didn't have at least 90 days of real writing days, where the first and most important thing I did was write quota/produce first draft (and that should be enough to produce a draft of a book or partial drafts of two books), but that a year in which I have 300 writing days is possibly or even probably a year in which I feel bored, grumpy and understimulated; a semester of teaching gives me a hunger for writing time that I can then really take advantage of, and I do not envy those who have undertaken the life of full-time fiction writer.