As a NaNoWriMo participant, I've been trying to meet a rather ambitious 1667 word/day goal. I've heard that professionals do a lot less, but the first two days of this month, I actually wrote 5000 words/day, so I know it's possible. Quantity over quality? What's a reasonable, sustainable word count?
It varies a lot - and by type of writing. I can do casual non-fiction writing fairly quickly, but more structured or formal work takes me longer (not just in the editing, but in the actual writing process).
The professional writers I know point out that burning out is a bad thing (as above: 1000 words each day - or even 250 words a day - is a lot better than 3000 or 5000 or whatever words once every two weeks.) Also that it's much easier to get started if you stop when you know what the next piece is (i.e. don't stop when you run out of stuff: try to stop when you have one thing left that you know. Then, the next time you sit down, you can start with that, and build momentum.)
An apocryphal story about Joyce
A friend once found him sprawled across his desk, a figure of utter despair. "How many words have you written today?" he asked him.
"Seven," the great man answered.
"But that’s good for you, isn’t it?"
"I suppose so," Joyce answered. "It’s just that I don’t know what order they go in."
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I think 1,000 a day is pretty good, if you're consistent. A day with 5,000 is possible, but probably not sustainable for most people. 1,000 every day is better than 5,000 once every two weeks.
This may be an extremely subjective answer, but i find it easiest to get a chapter or two done, rather than a word count. It means there is a complete section to get a family member to read over, and if there is spare time in my sitting, i can go through and edit it.
If you don't finish the chapter, just try finishing it (and the next chapter) the next day!
On the extreme side, Lester Dent was able to write a whole Doc Savage novel in about two weeks, perhaps less. We're not talking great literature here, but he is generally considered to have written the best Doc Savage books (all of them were published under the name "Kenneth Robeson", and they came out monthly). They weren't all that long, so figure about thirty-five hundred words a day, sustained, as the high end.
Some authors seem to be able to write a novel of maybe eighty thousand words a year, of good quality. That's more like three hundred words a day on the average.
No one can answer that question for you. You need to write every day and find the best wordcount for yourself. As a 4 time winner of NaNoWriMo, I can honestly say that 1667 words a day for me personally is way too much. It's doable, but if I had to go one more day, I'd go insane. 1667 a day burns me out, even without a day job. On the other hand, I've met a lot of people during NaNoWriMo that do 1667 a day with ease, while others even go for 100k or even 150k. It really depends on the person (and the story, how well planned out it is, how inspiring, how easy it flows...).
All in all, I'd advise you rather choose a range than a fixed number, or perhaps a bare minimum. There will be days when writing will not come easily, and it's better to take it slow than to force it. Forcing might be necessary during NaNoWriMo where deadline is tight and quality is not an issue, but I really see no need for it in every day writing, it unnecessarily lowers the quality.