I just logged in to one of the forums I've been participating in. I hadn't posted there for ages, and was curious to see how much I'd written. It turned out I had racked up 18 posts on various subjects. I copied all my posts to a file, which showed the word count: 1500 words.
So, that's quite a bit of material, on a range of subjects. And that's just from one forum.
It just goes to show that if you join these communities it's pretty easy to develop a lot of content. As long as you rewrite it substantially to make it unique in the eyes of the search engines you can use it again for articles and blog posts.