In our SEO Services, we request users to change information on their site at a minimum of weekly. This doesn't mean rewriting home pages etc, but it does mean adding new blog articles etc.
Think of it like this - would you visit a friend who hasn't had a new thought in several months (or years)? The same conversation every time? Probably not! This is how Google thinks too! Why list someone who hasn't changed anything on their site in "X" months, when you could list someone that has an interest in their site and keeping things fresh?
Again, you don't need or want to change things on your main pages every week, but it certainly should be changed every couple of months, depending on your SEO strategy and how your results are going.
Rarely do we suggest a complete rewrite of a page, but depending on what results are happening, something it's best to rip the bandaid off.