The developers recently announced that a "speed run" of KoA's main quest and every side quest took roughly 200 hours to complete. And that's with already knowing exactly where to go, and what to do for every quest. And then they questioned if perhaps they might have gone a little overboard.
The main quest alone is supposed to clock in between 30-40 hours, if you just focus on that. I have no doubt there will be people going for 100% completion. I suspect it will take well over 200 hours. I also suspect it will take well over 400 hours if they stop to harvest every plant they see. Even just outside the Well of Souls I was stopping every ten seconds because another shiny harvestable was begging to be plucked.
The game is a nightmare for anyone who can't walk past a potential lootable.
I know for a fact there's no chance I'll see everything KoA has to offer, but I like knowing it's there nevertheless. Who knows, I find the combat far more enjoyable than anything Skyrim brought to the table, so perhaps once I really get into it, I won't be able to put it down.