Paix can do 40 mins atleast, when it's less it's usually to add him in later for more effect.
There was a few games where he did 50 mins and looked fine and then others where he did the same and looked destroyed, It depends on game intensity.
When it comes to Niu, while he is fit when it comes to running, he might not be fit for decent minutes of middle defence which will put more strain on your muscle endrance rather then your aerobic endurance.
Besides this has been Paix's breakout year, he is still a bench hooker yes but as shown in Origin the game might be heading in the way of having your best hooker, being the interchange. it is definitely the position that can change the game the most if you bring fresh legs on.