How long/short it is depends heavily on a few factors. The game is designed so you can make essentially a brawler/fighter build where you use force to put everyone down and you basically run into confrontation and deck everyone. Or to play it stealthily so that entire missions are possible with zero kills and zero alerts, the stealth play is a lot slower.
It also depends to what degree you want to loot the levels, the game when played as stealth is a lot like Thief where you can't help but loot everything.
Lastly most (I think all) of the levels have multiple paths to completing the objective, usually there's the brute simple way of taking out the target, but there's normally a much more subtle and complicated way which lets you avoid assassination but is harder, especially if you're stealth and have to go back and forth through the open levels to fill the requirements for doing so.
If you want to go for 100% in the game then you need to get the achievements where you get zero kills and zero alerts for the entire game (which I did get) and that means a 100% stealth play through and it means finding all the passive alternatives to assassinations. That's a very rewarding achievement to get and I normally don't bother with achievements at all.
The game is definitely worth playing and unlike many "buy our DLC!" cheap cash-in games, The Knife of Dunwall and Brigmore Witches are pretty damn good that actually feel like old school expansion packs worth the money.
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