Question

Using the devel module I can see a lot of calls to cache_get() and cache_set(). After how long does a cached value need to be refreshed? Does the cache get invalidated every few minutes?

Answer 1

The module that is using cache_set sets the expiration in the call. Some things have explicit durations, others have permanent or semi-permanent lifetimes, based on the situation.

Caches get explicitly cleared when you invoke the method through the admin interface (or drush), or otherwise through the use of drupal_flush_all_caches or cache_clear_all.

Lately, I have been using a hook_cron to clear certain cache tables each night.

EDIT to answer comment:

To see which cache, I usually put this in a separate script somewhere:

require_once './includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
header("Content-Type: text/plain; encoding=utf-8");
$user = user_load(1);
print "Modules implementing hook_cron:\n" . implode("\n", module_implements('cron'));

To see expirations, examine the various cache tables in the database and look at the expire column. Modules can set expirations on each individual call to cache_set, so it can vary entry by entry.