In an interview with Polygon Bioware producer Cameron Lee said Dragon Age: Inquisition could take players in the region of 150-200 hours to fully complete.
Lee explained that 'people could burn through it and just do the main path, but we wanted to have a world that's fully immersive and well-defined for you to explore, discover and get involved in.' Speaking of the main story content Lee said, 'it normally takes 20 to 40 hours anyway to tell the story we want to tell.'
That's quite a gap in time to finish the main story, and with 200 hours for the entire content it looks like players will be in Thedas for quite some time. No mention of time for the co-op content though, which is bound to set aside many more hours for players to battle through.
Bioware have been doing some livestreams of Dragon Age: Inquisition lately, the first of which looks at the character creation tools and gameplay. Another livestream was broadcast yesterday which expanded on the first stream some more.
Dragon Age: Origins, the first game in the series is currently free over on Origin, so make sure you pick that up.