<p>The SE powers-that-be make those decisions.</p>

<p>I don't know their policy, but Jeff Atwood, in his role as an SE representative, <a href="https://meta.stackexchange.com/questions/984">asked a similar question on meta</a> three years ago.  It, and the ensuing replies, are part of the SE FAQ, so presumably it's a good guide as to what to expect.  Although it's not conclusive about the meaning of "regular elections," on the whole, "regular" looks like it depends on the community's size and rate of growth.</p>
