Searching habits change from time-to-time and customers may start using different keyword and keyword phrases when looking for your business services and products.
Therefore, you should also consider checking the effectiveness of your keywords for SEO, Pay Per Click and social media.
A "hot" keyword two years ago may not be the best keyword today.
Using a combination of the Google Keyword Tool (Google Adwords), WordTracker, Keyword Discovery and other researching tools, you should consider checking your targeted keywords every six months.
Of course, any time your company introduces new products or services you'll need to conduct keyword research to find the best keywords that closely match your needs.
Finally, changes in targeted keywords also need to be reflected in new Page Titles; updating other SEO elements and in link building strategies.