I get asked this question all the time. There is no actual answer as I have seen people such as Seth Godin write consistently every single day with one or two paragraphs, and then others writing long tail articles of over 2000 words once a month.
If you key objective is to keep the Google spiders happy, then we suggest a minimum of 500 words once every two weeks. We have found that longer articles (1500-2000 words) are more successful. Not only does this keep your visitors on your website longer (which Google loves), but the content will also be full of juicy keywords, which the Google spiders adore!
Guest blogging strategy explained is here: https://www.youtube.com/watch?v=-kCgou1gU6Y
How often do you blog? Do you have a guest blog strategy or do you write all your own content? I'd love to hear from you.