I think each 2.X patch is about 3 hours or so of story, though I've never gone straight through without getting side tracked on other additions. As others have said, 2.1 has a few spots that will take time waiting in queue (less so if you're a tank), and the HM primal fights aren't exactly popular (no rewards but they're in the trials roulette so they have some traffic), but otherwise it's mostly solo stuff and cut scenes.