Do moderators have a term limit after which they must retire?
Do we re-elect every moderator after a certain period of time?
When are people eligible to become moderators?
Moderators elected on Stack Exchange have the same term as the Pope: for life. They also have the option of resigning.
New moderators are elected as needed, which usually means when there are too many flags for existing moderators to comfortably handle. There's no fixed schedule, but we usually check with them around the time of a site's graduation anniversary. See also: Can we have more moderators?