For our Drupal 6 site, we've experienced such issue that logged-in users are getting logged out for around one week or several days.
Firstly I thought this might happen with Boost module, but it seems this issue happens as our test site that runs without boost enabled. We have already set cookie time long as following in settings.php
ini_set('session.cookie_lifetime', 2147483647);
ini_set('session.gc_maxlifetime', 200000);
This site is pretty big and includes a lot of custom modules/code, which make it difficult to debug what's going on. I greb the code, and no luck to find session_destroy code, or user logout code. Maybe I need to set session.gc_maxlifetime as large as session.cookie_lifetime?
Is there some way to debug this issue when users are logged out?
