"The Rule of 200 works like this: my document word count must increase by 200 before I am done for the day, no exceptions. 200 words is a modest goal. It isn't even an entire page of double-spaced 12pt font. It's a grocery list, an email, a series of text messages; it's a lot shorter than most of my ProfHacker posts (this one included). Sometimes it takes me 15 minutes to write 200 words. Sometimes it takes all day long. But no matter what, before my head hits the pillow for the night, the word count is +200." (Chronicle)
I guess in the summer or on sabbatical it should be more like 1000 words or more (plus the research that allows you to write these words).