I think he should stay as a manager as long as he can motivate the players. Only if he can't do that anymore he needs to go.
I have wrote before that I think that nobody can motivate a core of our starting 11 (Pique, Masch, Busi, Iniesta, Messi, plus to some extent Rakitic, Alba, Neymar etc).
Majority of our starters are emotionally drained after too many years of winning.
So, on one hand, I don't think that anyone can motivate THIS squad anymore, except changes in starting 11 (2-3-4 new young players who haven't won anything, so that we can get a balance of fresh blood and old experienced guys).
On the other hand, not only that no one can motivate the current starting 11, but this is Lucho's 3rd season.
As have been said before, all coaches ran out of ideas and lost the ability to motivate the locker room after some time.
Imo, one day (summer 2017 or summer 2018) when we'll sign a new coach, we will need a new long term project with 2-3-4-5 new starters (like Umtiti, Gomes and some new similar guys in the future).
The current starting 11 is not dead, though, but...
On our day, we can win 7:0 against any team.
But the problem is that these emotionally drained players will play hot and cold more and more often.
There will be no more 10-15 wins in a row, but more like: 7:0, 1:1, 5:0, 1:1, slip after slip more often.
And in a CL, we can easily have problems against good and younger, more motivated teams.
So, sacking Lucho in summer 2017 imo isn't a solution if a new coach will again play with Busi-Iniesta-Raki-MSN.
These guys may be best skills wise (or close to the best), but an actual performance is a sum of skills+physical shape+motivation, and our emotionally drained oldies aren't on top anymore on a physical and motivational level.
That doesn't mean that we should change all players, but some new blood in starting 11 will be needed.