A simple question, I hope. Do you need to refresh and change your metatags frequently to get more traffic from search engines? Simple. Right?
I've never seen any info suggesting changing meta tags frequently. It is important to keep up with what search terms are trending, etc. but you'd likely be better off creating a page on your site optimized for those terms rather than changing an existing page. But changing for the sake of changing would be very risky and ultimately counter-productive. Google, etc. uses those tags as a beacon to tell them what your page is about but it still scans the rest of the page so if you're changing them and it's not reflected in the rest of the page, that's going to look funny to Google if that makes sense.
Mama to DS (6/07) , DD (6/09) , and DD (07/12) ..
Changing the meta tags isn't as important as just making sure that it stays relevant to your site's content. If you are getting a good result from your current meta description, leave it. Don't fix what isn't broken. :) Also making sure that just your content stays up to date is important. I suggest a blog right on your site for this purpose. Search engines love blogs because they are constantly giving them more pages and relevant text to crawl and index and you aren't risking destroying combinations that may already be indexed and doing well on your other pages.