Most of the missions are 20-30 minutes maximum, especially later on once you get more powers. By that point you are pretty much unstoppable as you only need to string together 2-3 hits and you then have two "finishers" to use, just brand the first few orcs and they will fight for you.
Just finished, 93% in 25 hours and other than the last hour or so which was a bit of a let down it is a cracking game. A couple of the forced stealth sections were a tad annoying but not enough to detract that much and they did suit the game.
I expect we'll see a lot of games copying the nemesis system, although will be interesting to see how they do it since it only really makes sense if the lead character is immortal (or has great health cover).
I'll certainly keep an eye on the planned DLC and may pick it up too just to encourage the devs/publishers to do a sequel.