I voted for 50+ - because it will be
at least 50 chapters, but it will depend on whether he wants to do another story arc or not for much beyond that.
I am inclined to see this as the final grand hurrah of the manga, the climax, since every single character that has died/been defeated has some back in some shape or form (mostly zombie.) Naruto has already conquered his inner demons (literally) and finally has the emotional maturity necessary to take his place as the real hero. There's honestly not much character growth left for Naruto alone; any further growth will be his interpersonal relationship with Sasuke and/or (hopefully) Sakura.
However, if Sasuke escapes this war and becomes the Final Mega Boss (a role currently occupied by Madara) instead of the Familiar Boss (forgive the RPG terms here), then I can easily see another 150 chapters. Urgh. But for now, the
goal is saving Sasuke, and Madara is merely an obstacle to achieving that goal.
A lot of factors go into this besides the storytelling. Kishomoto may be forced into an additional arc by Shounen Jump due to the series popularity, or he can double down if he has a new idea for a new manga. Rumiko Takahashi put her foot down and finished Ranma 1/2 because she came up with the idea for Inuyasha, and Shounen Sunday finally caved and let her end it (and then take a much needed six month planning break.) I don't think they regret that decision now.