Single Player is 5-7 Hours

According to a Q/A video from Gamexplain, the single player campaign mode in Splatoon is approximately 5-7 hours long.

Honestly, this is about what I was expecting, so I'm not disappointed. It's about the same as most shooter campaign modes anyway, I mean the newest Call of Duty had like 4 hours right? And Titanfall's was pretty short from what I heard. Plus this is more of a 3D platformer with Splatoon's mechanics, so it's pretty nice, about the length of Super Mario 3D World, which didn't have a huge online multiplayer mode as well.

That's a good amount of time.
Not too long, not too short.

I guess that's a decent amount of time and they said it will take more or less time if your good at the game.