To a large degree, this system is based off a mix of my experience with a DB game I was in back at university, and then cross-referencing other "large scale / small scale" systems and works - especially the Total War series, which with its "Turn based strategic scale / real time battles" kind of operates at an Exalted empire-building scale (ie, above the character scale).
So, what would happen in the DB game is the GM would ask us what our plans for the season would be, and that'd set up the background activities and produce plot hooks for the session. That's why the Major Action/Minor Action stuff operates on a seasonal basis - and Trivial actions exist to allow players to just take time to get the team-up stuff done.
Do also note that Trivial Actions do not need to be a scene, if they're suitably trivial. For example, with
@Aleph, stealing treasure from the local prince if she's just doing it to get her hands on a piece of art or to acquire funds in the local currency doesn't need to be a major heist - that's just a one roll thing.
As a result of that, if your character doesn't want to take any Trivial actions and wants to focus on getting their major projects completed, that... just happens. Narratively, that's the story of your deeds going "Six months later, Inks looked over at her new dam with a contented expression. It had been hard work, but it was finally done".
For the question of the default, as it stands it basically exists at a practical level at the moment, working off rule of thumb for "how long something should take". A Minor Action is one that takes a few weeks - a Major Action is one that takes the best part of a season (probably 2+ months). As a result, in the current provisional model, the pre-Charm time to do such things basically involves the player saying their overall goal, and the GM breaking the goal down into sub-tasks which may or may not have dependencies on previous tasks. As a result, the default assumption is that the player doesn't have much control over the length of time the Tasks take.
Now, on the other hand, I can easily see space for such things - for example, if
@Aleph wanted to survey an area for manse-OK geomantic sites, I'd offer a Minor Survey or a Major Survey, with the Major having a reduced difficulty due to taking more time over it/covering a wider area and so being able to inspect more prospective sites.
Again, as it exists, the system is more of a loose framework to give a shared vocabulary for marking out blocks of downtime for "I'm going to be doing this" and also for allowing team-work in a more meaningful way than the canon teamwork rules. For example, in canon, your War-focussed Dawn caste is basically useless when your Twilight plans to build a manse. In this system, the Dawn can carry out the "gather the workforce" stuff because with his War things, he can justify using War to improve the physique of the peasants they hire through training so what begins as undernourished peasants, with a season of exercises and proper food, becomes a well-trained and drilled workforce (that the Dawn can later call upon when they need to raise an army). And that means the Twilight doesn't need to carry out the Gather the Workforce action, and can instead focus on Sourcing the Materials - except, wait, no, their Eclipse with their Bureaucracy can do that, which means the Twilight is freed up to Negotiate With The Local Elementals which ensures that the local elementals will cooperate with the building and won't try to subvert it. And so between them, they've put three Major Actions of effort into it, and by the end of the Season, the Twilight has a trained, healthy workforce, has plenty of marble and the required amount of jade, and the elementals are pacified and cooperative. Which means they've just got to build the manse itself, which is the work of a few seasons of labour which may not even need the party there if they have a trained supervisor who can handle overseeing the physical construction now that the elemental won't object.
(that is to say, they can outsource the Oversee Construction action to a trained thaumaturge or something)