I agree with @keii4ii's statement, but I'll take this:
"how long should THIS story be?
And extend it to "How long SHOULD this story be?" vs. "How long CAN this story be?"
Compare it to the saying, "Scientists spent so long trying to figure out whether they could do [insert crazy science thing here], they never wondered as to whether they should."
Yes, you CAN make your comic/series as long drawn out as you want it to, if you know what you're doing. But does that mean you SHOULD? Some stories get away with being long or are in fact better off so, but these stories are built around their length and have it work to their advantage (ex. Harry Potter is longer because it chronicles Harry Potter's years at Hogwarts, with the main central conflict being the return of Voldemort, vs. Lord of the Rings where it follows the journey of Frodo and his friend Sam trying to dispose a ring that is capable of dark magic, though the story probably took a while to unfold with the journey, it definitely didn't require as many years to unfold as the plot of Harry Potter did).
Figure out your central conflict. @joannekwan mentioned that she will have Demon House come to its end when the main character graduates (I read Demon House so this seems reasonable and makes perfect sense). In Harry Potter, Harry's journey came to an end when [SPOILER ALERT] he finally defeated Voldemort. [not really warranting of a spoiler IMO but whatever]. In my own comic, Time Gate, it has several conflicts that tend to evolve as the story progresses. It starts with Uzuki being told she's going to die soon, and shortly after, she finds out that a murderer named Springlock she tried to dispose of as a child is still running around, so she makes it her last goal in life to kill him before she dies herself. Though other conflicts arise throughout the story that stray away from the whole Uzuki vs. Springlock thing, by the end of the story, they still have a conflict that needs solving - they still want to kill each other, and everything that's happened up in the story has only helped further their own aspirations. It's pretty much a very long drawn game of Cat and Mouse with some Snakes and Ladders mixed in, and a bit of The Game of Life sprinkled on top
ending up with about 10 books total (if I don't decide to cut it down, as there are still two books I haven't figured out are crucial or not... they were a part of the original trilogy I came up with as a kid so that's the only reason I'm still considering them) Is it all essential? Some of it could be cut out, and of course there are still things I'm trying to fix up in some of the books. But overall, the story is one giant domino effect where every domino is required to keep the chain moving in the right direction.
When you're writing your story, ask yourself: "If I remove this scene, will the rest of the story function just fine? Is it 100% absolutely crucial to keeping the gears moving in the plot?" If you find yourself saying no, and not feeling like you need to be working it into the plot, then consider removing it. Though it is nice to have some parts of your story that are a little more "filler" (hell we've got a chapter coming up that's completely dumb for the sake of comedy relief), if you're adding these things in just for the sake of keeping your story alive, then you're basically forcing your very sick grandmother to stay on life support instead of pulling the plug and putting her out of her misery when the time comes that it should be considered okay to do so.
So that being said, asking the question "How do you keep a series going?" should only be asked when you're still trying to come up with your plot and tie it all up with a nice pretty bow (especially when you hit writer's block), not when you're trying to keep your plot alive simply so you can say you did. Though people may marvel at chapter numbers when they read manga like Naruto, Bleach, One Piece, and Hajime no Ippo (that manga has like 1000+ chapters, seriously), it's the story that sticks with them, and no one will say they preferred Bleach's story to Attack on Titan's or Deadman Wonderland's story simply because it happened to be longer-running and have more chapters (I mean, Bleach is just "Blech" with an "A" added in, OOOOOH SNAP)