That'll end up being the last battle in the manga, so considering the lengths of the current 3 when you uncoil them, I'll give 10 at the least, and here's the breakdown.
1. Long introduction/beginning before start
2. Few powerful moves; both reminisce and are impressed with the other
3-8. Fighting and flashbacks
9. Last of fighting; both are either about to die as Naruto said, or Sasuke finally changes
10. Either (as they're dying, Sasuke begins to see his errors and they finally are friends again; others show up; Sakura & most other girls cry, including Lee/others have sad/"good for Naruto" faces) or (they leave the battlefield and meet other main characters; Naruto kisses either Sakura or Hinata; Sasuke is forgiven when others see that he really didn't do that much)
The anime might shorten 3-8 to about 3 episodes, so about 8.