I've been writing articles for years but recently got into it a lot more. I am about to launch a blog and since I am in the process I've been writing articles to add content. I typically write 2 to 3 articles a day which averages to 600-900 words per article. Sometimes I go back to old articles I wrote and update and rewrite some of the content to keep it up to date.
I really don't know what the average writer writes but I assume it matters how much free time you have to write.