I believe Polygon said the campaign is about 3-4 hours long, or faster if you rush. Seems shorter than TTK's
*long post warning about campaing, no spoilers tho*
It took me about 2 and a half hours to do it.
The thing is that, the campaign is super short. About 4 to 5 missions.
The problem however is that there is no emphasis on exploration. There is no little collectathon feature as you breeze through it thats going to make you replay the whole thing down the road. Or some little element that adds replayability. Like, Nothing. Nada. Zero.
You just point and shoot your way out through waves and waves of enemies.
The good thing that I will highlight tho, is
the way Rise of Iron conveys the story to the player and it is by very far, better than the base game and a bit better than The Taken King, I will say.
It adds solid character development (something that was always hit and miss in vanilla Destiny) to new characters such as Saladin (The Iron Lord) and expands the lore more and does it suprisingly well.
So, I'm glad to see Bungie is adressing one of the weakest point of vanilla Destiny, and doing a mostly solid job which tells me its going to bode well for Destiny 2's campaign as an RPG. While the story-telling still needs a little bit of improvement (when the game ends, it doesn't explain much the perks of being an Iron Lord or their impacts, the Guardian Tower's reaction to it, overral world recognition, etc)... But I will say this is a great foundation to build on going forward. I can work with this.
Destiny 2 needs to go all out on exploration, and I firmly stand by this. Thats pretty much the only big Con that still persists on these DLC's, and it hasn't seen an improvement whatsoever. It was bearable on TKK because the maps were much more open and contained varied enemies to keep you hook for a certain amount of time, but it became pretty clear to me with RoI.
All in all, campaign felt mostly as a cinematic Call of Duty campaign, and not as an Rpg-centric campaign. Very short, maps were pretty small (except Plaguelands) and like I said, sadly no emphasis was placed on replayabilty or exploration.
Very happy about story potrayal though. Bungie is learning slowly but surely, how to tell a story and develop characters.