500-1000 words

 Why think of writing 500-1000 words a day on serious writing days? 

Clearly, 2000 would be too much. The article will have 6000 words, so doing it in three days is not so realistic. I am not saying I could never write that much in a day, but it would be taxing. 

100 words would be too little. I can do that by sitting down for 10 minutes. But I don't want to write the article in 60 days, because to get 60 days might take six months, given the inevitable interruptions in life. Maybe there's something else you need to write and so you leave off your main project.

 If you aspire to a good number, even if you don't reach it, you will still make some progress (300, 400...).  Just as I aspire to walk 10,000 steps a day: if I fail and walk only 8,000, then I am still better off. 

1000 words takes about three hours. Then I can feel good about the amount of effort as well as the number of words. The caveat is that at the end of the process, more time will be spent rewriting, so that word count will not rise as quickly.  At the end you might want to revise and perfect the paper in another 3 days.