Exactly as long as it needs to be. 
Not one episode missing, not one episode of filler.
It sounds like a joke, but I’m serious lol, the story should be as long as the plot.
But personally, I won’t get into anything if I know it’s going to be over 50, I’ve seldom seen any story, even the best stories in the world, be able to carry a plot for that long without it starting to wind. Some of my favourite TV shows or irl books, I dropped them somewhere around book/season 5-8, even those that I was binge-watching/reading during book/season 2. 