"Regular" is probably not the best wording here, because elections don't happen during the same week or two each year, unlike, say, the US presidential election. It is true that there is likely to be one (or maybe two) election(s) each year, though, simply because of a few factors:
- Sites generally grow, and so more moderators are needed.
- Time constraints might become an issue for one or more of the moderators on the team, and so some more help is needed. Things happen, and we all experience temporary hectic periods once in a while.
- Most moderators don't moderate for life (although, of course, someone's got to be the first), so retirements happen on a semi-regular basis. Going to a site's election page (i.e.
sitename.stackexchange.com/election) will show you how long, in general, mods last.
Taking all of these into account, as well as any unusual scenarios that might come up, a good sized site will probably have an election or so a year, so it becomes regular by default.
Also, When do moderator elections take place? might be a helpful read.