Definitely ending. It doesn't have to be epic, it just has to be good - it should flow logically from the rest of the story and wrap everything up in a satisfying way. And "satisfying" doesn't have to mean "happy and all the popular pairings get together."
When a story ends, it's sometimes because it was cancelled or the author lost steam. This isn't that common in manga, but it does happen. Maybe you read some of those.
In most cases, when a story ends, it's because there is no more story to tell! If they went on, it wouldn't be anywhere near as interesting as the original story.
The stories that go on forever that you've seen are in most cases the result of one of two things:
1. The story proved so popular that the editors/authors didn't want it to end so they could milk more money from the fans. This is very common in manga (for example, Bleach). Similar forces are at work in many superhero comics.
2. The author was influenced by stories affected by #1 but didn't realize that these were negative things and imitates them as if they're intentional. This is more common in webcomics.
Always, always have an ending in mind when you start a comic. You might go another way later, but even then, know your ending. Even if it changes.
It is better to finish a story and leave the readers wanting more than go on forever and lose their interest, or, even worse, not finish at all. Remember, comics are a lot of work. If you want to make a good one, be prepared to spend hours writing it, and significant time on the artwork (after all, what's a good story if the art doesn't draw anyone into reading it?). The longer your story is, the longer it'll take to draw it, the higher the chance of you not finishing is. Even pros lose steam.
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