How often (on average) does drupal core need updating?
There is no "rule" but update as soon as it's released.
Usually Drupal core updates (bug fixes) are released once in a month but in case of a security vulnerability, there will be an immediate release.
I usually leave Update Status module enabled and configured to check for new releases every week.
Read more about Drupal core release cycle here. (This is about major Drupal releases, eg. 6, 7 and upcoming 8)