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 an Aegir server that hosts Drupal-only sites, and makes our upgrade process speedier and more consistent. It allows us to set up secondary platforms and full test sites prior to bringing them live.