I've just acquired a digital copy of Root: the RPG, and I'm going to master it with a group on the following month, but I'm not sure how long a session should last. I know that in a "typical" D&D sessions last around 6 hours or something if you want everything to be done as is. Here, I don't get how long it should last and how to space events to make it nice and enjoyable.
Once the game is running, 4 hours is a good benchmark for a session.
Finally, you'll need some time, probably between 3 and 4 hours. The first time you play, character creation is likely to take up to 2 hours to fill out all the details, set up all the connections, and so on. From that point forward, a good session or "episode" of play can take about 3 or 4 hours to play out, so try to plan for at least that much.
-- "Preparing to Play", Root Core Book pp. 35-36
The general rhythm of accomplishment in a PbtA game usually fits well in a standard 4-hour convention slot. Usually this breaks down into 2 hours to make characters and learn the game through play, and 2 hours of more practiced play after a mid-session break.
In Root's case there's an obvious pacer element to sessions - everybody's got drives to chase for advancement, once per session per drive. Regardless of what the band may have been "hired" to do, using the setup and accomplishment (or failure) of drives is a good way to make sure you've done enough for everybody. 2 hours is a reasonable payoff time for one drive for everybody, and 4 hours is a good target for both.
However, there are other considerations.
Making the initial woodland setup is a nice little "How To Host A Dungeon"-style exercise, but probably one you'll want to do yourself before play begins, unless your playgroup is really really into Root and would like to get het up about riverbanks and a catfight with some birds.
If this is just meant to be a teaser session to see if your group likes the game, consider "skipping character creation" - that is, showing up with premade characters and a scenario in a clearing they all fit into. You'll still have to do a rule teach and that usually breaks down along the character creation lines anyway, but you've already made all the relevant choices. Make a map (the example one in the book is fine), pick a clearing, and assemble about half again as many characters as you have players. When you pick their drives, also provide some notes about how they can follow up their drives in the clearing your game starts in. This gives the teaser more of an "established series" feel, where people already know something about the state of play and can hit the ground running. If you go this way I'd recommend Milltown from the example map. A base on the Eyrie expansion front that fell to a civilian revolt, right next door to a different suit's Woodland Alliance base, where the Marquise has no direct access but every motivation to slip some vagabonds some cash to be a big distraction? There's some good stuff there.
If you're the type of playgroup that makes big blocks of time for each other, of the "all day" or "weekend vacation" variety, you might be tempted to try and get multiple sessions in, but be careful with that. PbtA games tend to make big demands of both player and GM engagement, and one 4-hour session might be your limit for the day, at least without a long break to recharge. If you do play multiple sessions per day, it might be a good idea to only allow the character-revision part of end of sessions at the very end of a day, unless somebody has a really solid plan. That's the sort of thing that's really best treated as an open question until the next time you meet.
If you're the type of playgroup who doesn't have a lot of time for each other, you can probably get a satisfying mini-session out in around 2 hours*, chasing up one drive for each character. If that's your regular cadence of play I'd consider only allowing character revisions every other session, since you're getting through less gameplay with the revised characters, but still clearing drives and such at the end of the 2 hours.
* Always leave time for aftercare. Not just your end-of-session move-making, but an actual cooldown period after the game and before the rest of your life. Take some time to reflect and let everyone talk about what happened, get feedback on the game while it's still fresh in everyone's minds, and in general give people time to take themselves out of the headspace of the knifecrime raccoon, who is ultimately no more welcome in our polite society than they are in the Woodlands.
About 4 hours
The game page you linked quotes session times as
2-4 hour run time | Rated Everyone
On to the "curated play" program offered and sanctioned by Magpie Games, Root's creators, there are currently 5 sessions on offer: 4 of them are 4 hours long, one of them 5 hours. So in practice they seem to really be better set at the top end of the 2-4 hour range, or about 4 hours long.