If no lists are required, cron queue may never empty.

If you have 10K users, no required lists and set mailchimp cron limit to 2500 per run, you will be left with 7500 users in the queue permanently.

This is not a major bug because having a full queue won't affect the module if there's no required list.

Mailchimp cron selects the list of users to update from queue (first 2.5K of 10K), then retrieves required lists. Only if it has a required list will those users then be moved out of the queue.