Been playing beta for a couple months now. Hunter, DH, and lock all 110. The new daily system takes a shitton of time, so you probably won't have more than 1-2 toons active at any given time until you cap your reps and artifacts out.
That said, you get not just one, but *TWO* garrisons in this expac. Your class hall, and the town you set up in the final regular zone. You will spend vastly more time in the latter.
Legion brings back the rep grind fever you all know and love from TBC, along with shitty assfuck 'mythic' mode which combines the annoyance of having to manually form groups with the inconvenience of having to travel all the fuck over the map to get to dungeons, because fuck you that's why. The shitty devs that got their asses handed to them over no flying in WOD are back in action, waiting to drive yet more subs away from WOW. If they don't add mythic to lfd system before first quarter is over, I am betting they will be under 2M subs.
Back to reps and that final zone. Suramar has a lot of fun stuff in it, but once you clear out all the stuff you have at any particular unlock stage (locked to rep) you have extremely limited options. You will only have 3-4 WQs in the zone at first, with tiny amounts of faction, so you will be farming this zone for 2-3 months before you get to exalted, which is supposedly when you will unlock the raid area of the zone. That's unconfirmed, but I have personally confirmed that the two Mythic only dungeons in the zone, unlock at 8000/12000 into honored. If you have a tradeskill, you will be forced to spend time in Suramar. Raiding and dungeon progression? Suramar. And there is a fun minigame you get every other day in Suramar where you lead a horde of mana starved NEs through an instance to try to get to the other side. Personally, I have had a lot of fun with it, but I can see where others might not. One thing you get from that is a bank for your little rebel town, so it has a benefit. And it is a nice boost to nightfallen rep when it pops.