Thanks for the ask!
Since this question is pretty general, we’ll give a few suggestions. If you’re looking for gritty realism with unpowered characters then try to keep it under 2 pages, but remember you’ve got to provide enough detail that show the fight is clearly choreographed. You’ll need to be able to convey the action to the reader so that they can follow along, while also being able to keep the narrative tension high. The longer the fight, the harder this is.
For most powered characters, superheroes, etc, you can double it to four pages. Though, I strongly encourage you to still look for a way to end it quickly. There’s nothing worse, from a narrative and reader perspective than a fight that goes: and then, and then, and then. It can be easy to get caught up in the flow of the action, both written and visual, and become excited over your creation. This is natural and understandable, after all, you created it! But I encourage restraint.
Also, it’s a good idea to check out artists in the genre that you admire and see what they do, not even in art style but just in planning and conservation of the action in the way they tell their story.
And remember, the action always serves the narrative, not the other way around.