For the worldbuilding I use, I'm mostly standing atop the shoulders of giants: fluffynexu and inquisitorhotpants. The latter has an entire post about populations of the Sith Empire (here), with the conclusion being that there's about eight-hundred billion Force sensitives within the Empire, those who have been trained and those who will be.
According to Dee's worldbuilding, about a hundred billion of preparatory school age (5-15), two-hundred billion attending an academy (15-23/25), and five-hundred billion serving the galaxy.
There are, canonically, many academies across the Empire. One of which was written about is the Odacer-Faustin Sith academy, which was destroyed eight years after the Treaty Of Coruscant, two years prior to the start of the game (source: Star Wars: Red Harvest).
I'm inclined to agree with Dee's assessment on academies in the Empire. One academy per system capital, with massive elite academies that service millions of acolytes at any one time. Most academies have upwards of thousands, tens of thousands, hundreds of thousands of students, with the elite having a potential for millions.
As for calendars... the answer is rather simple, actually. Though I have no doubt that there's local calendars for every planet, the Sith Empire itself would run off a single universal calendar across every planet. Local time verses Standard time. There is an argument for Korriban to serve as a basis for Standard time: it's the Sith homeworld. However, I feel like the Empire would use either Dromund Kaas or Ziost for the basis of their Standard calendar.
Dromund Kaas is their capital, and has been for over a thousand years. Compared to the Korriban calendar, it's much more reasonable with a 24-hour day and a 312 day year. Dee created a calendar based off Dromund Kaas (here).
I went with Ziost (here). It's the other Sith homeworld, served as a major hub for the Sith people for over twenty-two thousand years. It was the capital of the Old Sith Empire, and the Reconstituted Sith Empire is a direct continuation of that Empire. Not only would it be a source of consistency for the recently exiled Sith, but it'd be a sort of promise that they would return, eventually, and reclaim their home.
So, for my worldbuilding, it doesn't matter where you are in the Empire, you're going to use the Ziost calendar as the Standard. This goes for the academies, too.
The actual mechanics of the academies are tricky, though. Both the Inquisitor and the Warrior begin their story with a set coterie of six acolytes, slave-caste, with an additional acolyte from an older Sith family; they both begin with a Sith Lord who has already claimed said coterie; they both have six trials to complete over the course of their training; and acolytes are picked off one by one with each trial.
It's very different from how we're introduced to Sith academies elsewhere. In both KotOR and the Bane Trilogy, we're given academies that are more a free for all with all acolytes vying for the attention of a Sith master, not a trial to be seen, and with students actively cheating or otherwise sabotaging each other.
Ultimately, it depends on how you interpret the game, how much you want to keep, how much you want to change. I like some of the ideas presented in SWtOR, like the overseer-coterie system. A Sith educator has a small group of six or so acolytes who they dedicate their time to teaching. I like the idea of trials defining education.
It sucks that we're given no real time-scale to look at. Like, at all. We very rarely get any idea of what an "average Sith's education" looks like in any source material. Bane's the closest, not even a SWtOR Sith, but his education isn't "average," and only lasts for about a year or two. After that, he somehow becomes powerful enough to challenge his teachers for one reason or another.
I went with three years for a variety of reasons. The first is that I don't think Zash would be willing to wait too long for an apprentice, even if she is following her visions. That means that the Inquisitor, at the very least, has a rather short education cycle.
The second is that I feel like the Sith academy isn't about creating fully fledged Sith. The education is well rounded, but it's more foundational than anything. Their proper education begins under their Sith master, or it's supposed to at the very least.
I also like the idea of the trials defining an acolyte's experience at the academy, and two trials a year seems reasonable, all things considered. Three years is the lowest I could go while keeping Trials at a regular, mathematically pleasing interval.
I could do six years, too, but six years is a lot of time and, as someone writing about an acolyte's journey from start to end, I feel like it'd be a lot harder to write. A lot of skipped time, all of which wasted unless I get really creative.
I'm open to being convinced otherwise, but that's how I envision it.
Acolytes begin their education at 16 (though, former slaves can and do pop into the academy at any age over that), and a little detail I like is that acolytes who haven't passed their first trial go by "neophyte."
As it stands, I think most of one's time at the academy is spent, well... learning. There's physical conditioning, martial training, military training. Students will be expected to get their pilots license, and have an understanding of philosophy, history, and the laws of the Empire.
Specific "scenes" I can imagine are acolytes doing wilderness survival, thrown into the wild and expected to return to the academy / civilization on foot. There'd be mock-battles and war games with acolytes expected to take on the roles of grunts and commanders. I also think that any trial an acolyte is set to partake influences their education. When Harkun hands you off to Zyn, your Inquisitor ends up studying torture and interrogation for months prior to actually doing your trial and interrogating Alif.
All things considered, I feel like there's four endings to an acolyte's time at the academy.
They pass their trials and are taken on as an apprentice.
They pass their trials but aren't taken on as an apprentice, instead being drafted into the military spheres to earn their eventual lordship in service to the Empire instead of service to a Sith Lord.
They fail their trials yet live, being allowed to re-enrol at a later date, or else be shunted off elsewhere to serve the Empire, but not as Sith.
Most acolytes don't actually die. It's entirely possible, but the academies aren't meat grinders... for most. The ones who have to watch out are those who were slaves, who face discrimination by their overseers, who aren't given the privilege of prior education at a preparatory school, and who are legally owned by the Korriban academy until they graduate.