The basic game is for 1-4 players, although there are expansions that let you play with up to six.  

With two players, you really have to play the factions of the Marquise de Cat, and the Eyrie, although there is a method of playing all four factions with two players so that you can choose the more difficult factions, which include the Woodland Alliance and the Vagabond.  There are more factions if you get the expansions, but we have not delved into that yet.

It took us about an hour to learn how to play by reading rules and slowly working through the walkthrough booklet and cards.  I think that if you watched one of the video tutorials, you could self-teach in about 5-10 minutes.

We found that a two-player game can be completed in a half hour, which is wonderful because you immediately want to try again with a new strategy or even switch factions to see if you could win where your opponent failed, or improve on what they did!

I think the dynamics of the game change drastically when you add a third and fourth player.  The game probably becomes more adversarial, since someone getting close is jumped on by all of the other players.  That means you have to either be sneaky and sit in second place until you see your moment to foment an uprising, or you have to convince everyone that you are weak and they need to band together against another player.  

Like many games, the two-player dynamic that Cally and I see is different from the multiplayer version!

In fact, online reviews say that multiplayer games of Root can last for more than 2 hours, which seems crazy once you learn the factions.  The problem is probably that you need to know the other factions really well and be able to anticipate what they will do to get points on their turn since that will affect how you end your own turn, or even how you act during the turn to prevent being overexposed for example.

In our second game, Cally was able to look ahead a couple of rounds and see a strategy that I could have prevented by consolidating my forces.