As of today (22 June 2017) the last election was held in December 2014. A list of all moderator elections can be seen here.
On all Stack Exchange sites, elected moderators serve until
- they resign, or
- Stack Exchange decides that they are not worthy of the position (e.g., by violating the moderator agreement), or
- their inactivity level reaches some critical point (on most sites I believe simply logging in once every several months is enough, though Stack Overflow has much more stringent minimal required activity levels for moderators), or
- (and I am uncertain if this has ever come about on any Stack Exchange site) a moderator action review process results in a decision to remove them.
So why has there been such a gap in elections (which I imagine is the real question)? Quite simply, there hasn't been a need for new moderators. There have been no resignations or moderator removals since the last election (and that election was held, in large part, because of these then-imminent changes). Also, the work-load on the moderators has not noticeably increased in the intervening 2.5 years. Were either of these things to change, I am certain that an election would be scheduled fairly quickly. Judging by the last three months, the four really active moderators (who have collectively handled more than 99.5% of all moderator flags in this period) seem to be able to handle most issues in a timely fashion. As one point of reference, the average flag handling time during this period is about 90mins, which, if I recall correctly, is substantially less than prior to the last election.
(So, if you want another election you can try to substantially increase our workload by raising flags — or causing flags to be raised, but I wouldn't recommend doing things to attract flags.)
I will mention that Stack Exchange is planning to implement a system which I believe would lead to more "soft resignations" of moderators, and hence more frequent elections. I'm not willing to divulge any details I've been told, since (1) the system has not been finalised and is still subject to change before implementation, and (2) the info I do have comes from the Teachers' Lounge, and copying stuff out of the Teachers' Lounge is a very good way to have Shog remove the still-beating diamond from a mod's username.