Drupal core releases same-version upgrades every 4-6 weeks, and community modules sporadically. Major releases (Drupal 6 to Drupal 7) are less frequent. For our Drupal clients, we provide core and module updates to existing Drupal systems. This can be a light upgrade between versions (6.10->6.11), or a more major version update from version 6 to version 7.
Drupal code is not backward compatible, and upgrades can break a site. We have done thousands of Drupal version upgrades. There is no substitute for this experience— we know what pieces tend to break, and what to watch for.
We maintain a Rackspace server and deployment system that hosts Drupal-only sites, and makes our upgrade process speedier and more consistent. It allows us to set up staging platforms with the touch of a button and full test sites prior to bringing them live.