How often should you update your content in order to maintain a decent page rank?

The answer is simply: as frequently as possible!  From personal experience I have found that neglecting to keep fresh content on your site can cost you dearly in the page rank area. 
I have had PR5 sites drop down to PR3 after only 3 months of neglect, and believe me it is much harder to gain PR then it is to lose it!

This does not mean that you should go out and fill your website with spam or junk articles, a decent PR is not only based on how frequently new content is added but also on the quality of the content.  You should try to avoid using ezine articles and just spend the extra time creating unique informative articles yourself, you will have much better success at gaining PR this way.

Ultimately people are going to visit your website based on your content, so don't fall in to the trap of thinking that your site has enough content to sustain a good page rank on auto pilot, you need to keep updating and adding fresh content in order to be successful!