Fight scenes should be treated just like any other scene--make it as long as needed for the emotional impact you want to tease out.
As fun as shonen manga are, the one thing that peeves me about them is the heavy focus on technicalities in their fights. They'll discuss how a certain character exploits theirs and other powers, this and that--however, they sometimes fail to get me invested in an emotional level.
I remember the first time I read the closing chapters of the first Tokyo Ghoul manga.
As cool as the fight scenes were, I feel the best thing about them was their extreme relevance to Kaneki's development, as well as the progression of the story as a whole--it felt like they pushed the plot forward, instead of stopping to present to the reader a mere spectacle.
If anything, if you're going to have fight scenes, make sure they're important.
