Playthrough last 15~150(not really sure how they reach that number) hours.
In my case, my first playthrough has lasted a really long time (110 hrs so far at beginning of Act 3) in large part due to the fact that I keep dying. DA2 Nightmare is certainly more challenging than DAO Nightmare.
Since starting the game, I have developed better strategies for staying alive through tough battles, but it took quite a long time to get through the demo area and then Act 1. The upgraded version of Assassinate certainly helps with reducing the duration of boss fights when they are properly debuffed and brittled by my mages.
Personally, I like this sort of micromanaging gameplay and I still play this way, pausing constantly and directly controlling every party member. I have seen Nightmare mode YouTube tutorials, however, where some players set up well-planned tactics and only control Hawke, without pausing, even on the most difficult boss fights.