Depends on whether you're talking about adding brand new content or whether you're talking about simply "tweaking" existing content to make it seem "fresh."
And whether you're doing this because it's good for your visitors or whether you're doing it just because you think it's good for SEO.
And on what kind of site it is (as mentioned, if it's a blog, you need to add at least something fairly regularly; a more static type of site, not so much.)
As long as your existing copy is still relevant and still converts well, there's no need to go tweaking it just to get the spiders to visit that page more often. Test to see if there's a more effective wording, yes, absolutely, and if you find new wording that results in more revenue for you, by all means make the change, pronto!
But tweaking just to make older content appear to be "fresh"? Waste of time, IMO. Being spidered more frequently doesn't mean you'll rank any higher.
When you look strictly at the words on the page, Google doesn't favor "fresh" content any more or less than "aged" content. What they prefer is high quality content.
As to adding new content, as long as your new content is useful and informative for your human visitors, by all means add it as often as you like. Anything that's done for the benefit of your human visitors (you know, the ones who -- unlike search engine spiders -- carry credit cards) is a good thing, IMO.
But there's no need to go adding "content for the sake of content" or worse yet, "content for the sake of SEO."
Diane Aull - NineYards.com
: Helping Businesses Do Business Online
Whether you think you can, or that you can't, you are usually right.