Pro tempore moderators are chosen very early, and then they almost 100% of the time remain in power until graduation, which can take several years.
I don't have any stats on this but I'm guessing it's not nearly 100%. On one of the sites I moderate (I've been a pro-tem for two years there as of writing) it's definitely true; all three of the moderators have been the same for the full two years - but it's an extremely quiet site with very few moderation "issues". On the other site I moderate (for almost a year now), we added two moderators to flesh out the compliment to five because the starting team really needed the help.
The longer the site is around, though, and the more active (or possibly problematic) it is, the more likely it is that moderators will be either changed or added. When sites are in Beta, new moderators are chosen by the Community Team (the "CMs") and they are appointed until:
- the site "Graduates" and an election is held where all of the moderator positions are up for election.
- they choose to step down for their own personal reasons
- they are removed by the CMs (this is very, very rare)
There are absolutely ways to address whether a moderator needs to be replaced for some reasons but it's pretty rare to need to force change on the moderation team. While moderation can be complex on some sites, it's very easy, rote activity on others.
I've been around for a few graduation elections and, if the current pro-tem moderators nominate themselves, they quite often win - or at least a few of them do. It's not universal. Over the time a site is in beta many very valuable and active users can gain a lot of reputation and be seen as good moderator candidates but lots of users put weight in the fact that someone has already been putting in the time and effort to moderate and already knows how to do it.
In every area of society I know, people in power change once in a while.
Well... two things here:
They can... there's a reason US Senators and Congresspersons don't have term limits... and other roles in society do. There's a balancing act between having people with experience and having a "fresh face" in office. Yes, they're reelected on a regular basis (6 or 2 years) but it's pretty common for them to face little opposition for their seat.
Well, moderators have "power" but they don't have power in the same way elected officials do. Moderators don't actually decide policy on the Stack Exchange Network; the users do. The diamonds after our names set us apart but more in the way wearing a police badge/uniform sets an officer apart... and that's what we do - we enforce the rules that the network and our specific sites put in place.
So, moderators are more like a police force/firefighting squad (or garbage collectors) than they are like elected officials, despite the fact that they're elected/appointed. As such - much like the police, fire, and garbage teams - we do our jobs until we get a better offer/get tired of the work... or if they fail to meet the expectations of their position, which is very rare.
Why don't we allow pro tem moderators to change every year or so?
To take your statement purely literally - we do. We allow moderators the choice to step down whenever they wish. If someone accepts the position and then gets a new job and can't put in the time, they can step down and will likely be replaced with a new face.
To get at what you're really asking... elections right now don't happen on beta sites (excluding one test election on Parenting), so moderators are chosen directly by the CMs. They're pretty busy doing a huge amount of work, so asking them to re-appoint the moderators on 95 sites every year takes their time away from other stuff, particularly when the vast majority of those sites don't need or want a change.
If we did have actual elections, know that the election process isn't automated in any way, it's a bit creaky and semi-broken and takes a lot of babysitting by a small team of 10 people (those CMs I mentioned earlier). There are over 170 sites on the network. The election process takes three weeks from start to finish... that's over ten elections running at any given time, and that's without breaks for holidays, which are always bad times to have elections.
Annual elections would mean having rolling elections going constantly across the network with (if I had to guess) very little change in the moderation teams unless you forced new moderators to be chosen by term limiting the existing ones. On top of that, the way the current election system works plays nice with awarding a few winners (1-3) but probably wouldn't do too well with much more than that. Electing the Stack Overflow moderation team of 26 people annually would be asking for trouble.
And, well... the way the current system is ... works. The moderators act as checks on each other and they're there to enforce site policy, not dictate it. If they can't uphold that policy, they may end up stepping down or being asked to do so. New policy ideas can always be suggested by any user on the site and if the users come together and agree that the policy is good, then it's up to the moderators to enact that (assuming it doesn't completely break all of the Network's rules).
Surely these won't necessarily be the moderators for the entire duration of the beta (which may last several hundred days), when might they change?
So, to answer your question to Robert, unless there's some major problem or they choose to step down, they will be the moderators for the duration of the beta (which may last several years).
I'd like to add a small note... while some sites may have no problem replacing or adding new moderators, many do. Some of the elections I've participated in, either as a nominee or voter, have had very few nominees for the number of positions available - and there have been beta sites that have closed down because no moderator candidates can be found. Forcing sites (through term limits) to change their moderation teams comes with the potential downside of running out of people who are interested in the position who haven't already been pushed out.