The Drupal CMS releases a major new version of their software roughly every 10-12 months. With these major software updates comes the latest technology, increased functionality, increased ease of use and increased performance. As only the two latest major versions of Drupal are supported with bug/security updates we recommend investing in a site update every 12–20 months.
Staying current with the Drupal system will ensure that your website technology is leading edge. You will often find that each version is better than the next and carries with it additional side benefits. Moreover, keeping your technology current also presents long term savings as your business will no longer need to pay a larger sum for a complete website overhaul each time you want to do a redesign.