@amoeba There were elections in March 2011 (electing chl whuber, mbq), October 2013 (electing Peter Flom and cardinal) and March 2015 (Scortchi and myself); i.e. gaps of about two and a half years and then about a year and a half; there's no fixed schedule.
If some moderators were to step down or something like that, it would likely prompt an election (and that could happen any time). Of course, as the site grows, there would at some point be some need for an election eventually in any case. If there was a strong feeling from the broader community that one was needed for whatever reason, that would be a reason for us to ask for one.