From the help page Who are the site moderators, and what is their role here? (emphasis mine)
At Stack Exchange, we believe moderation starts with the community itself, so in addition to all users gaining privileges through reputation earned, each site has moderators elected through popular vote. We hold regular elections to determine who these community moderators will be (except in the case of new beta sites, which have moderators pro tempore, who are appointed by Stack Exchange staff). Moderators are elected for life, though they may resign (or, in very rare cases, be removed).
There's a post on meta.stackexchange that goes into a little more detail about when elections are held.
Basically, it appears that
- A community starts with some number of moderator positions available (we're currently at 3, which looks like the default)
- The StackExchange community team can modify the number of moderator positions available according to the work load (consulting with existing moderators)
- The StackExchange community team reaches out to moderators every year around the anniversary of their election
So, it appears that an election is only held when there is a need for additional moderator(s). Elections may be more regular on larger sites.
It took a good bit of diving through our meta here, but here's some additional information:
- SFSE was proposed around July of 2012 by LaceySnr ( http://area51.stackexchange.com/proposals/37589?phase=beta )
- Private beta began around August of 2012, and it looks like the public beta began not too much later
- We were in beta for a little over 2 years, mostly due to a general StackExchange redesign and the need to resolve IP issues with Salesforce ( When are we graduating and Coming Out Of BETA? )
- The first (and so far only) election was held in September 2014 following our graduation from beta (this meta appears to be the first mention of graduation: New Design Launched )
- Two of our 3 current moderators (metadaddy and LaceySnr) were moderators pro temporae during the beta