Anything up to 10/12 volumes is ok… but I prefer shorter works in general. I just like to know the story will end in good time and not be dragged out endlessly… it’s one of the reasons I will never pick up One Piece.
My habits are somewhat similar. I have a few series that run 10+ volumes, but none that I own in full. (Well, I do own all of ふしぎ遊戯玄武開伝 in English, but I’m thinking of Japanese editions here, and for that I only have the first volume.) The only one I’m confident is still in print is ねこねこ日本史, and that and 玄武開伝 are the only two I have any interest in continuing.
Heh, that’s kind of how I feel about Yona of the Dawn. “Oh neat, a fantasy epic with a proactive female lead…wait, HOW many volumes? So far?!” Maybe I should see if I can check out the English edition through the public library. (Also, コールドゲーム and 虚妄の女王 scratch a similar itch while being much shorter, though the former is an ongoing series.)
Agreed about preferring finished works in English (though the translated edition doesn’t necessarily have to be complete yet, as long as the publisher has a track record of finishing what they start).
In Japanese, I have a number of series that I have 1 or 2 volumes of (but run longer) - I’m working on catching up as I get a better feel for which series I want to prioritize. Most of them are in the 3-7 range, and at least two have recently ended.
In reality, in Japanese, I’ve mostly read series that are 5-10 volumes but I don’t think I’ve finished most of them. Some series are not yet complete, but mostly I haven’t finished them because I spend most of my reading on JoJo and it’s taking me time to get through the 131 volumes that currently make up the main series.
I prefer medium-long series, long enough to tell a good story. I usually don’t mind them being lengthy, but as long as it’s like that for a reason and doesn’t get tiresome.
Some examples of series that were/are heavy for me are 電波教師 or 山田くんと7人の魔女. Those are cases were the main story ends around volume 15 but the author comes up with another main story and then glues both of them together.
For example that’s not the case in other very long mangas like 名探偵コナン(for now, I’m currently at volume 29), かぐや様は告らせたい, Dr. Stone or even ニセコイ.
I also think that short series are interesting, but they usually don’t leave a great impression on me.
I dunno that I have a preferred length. Maybe a slight preference for very long, but only if the story is Good. My favorite series 暁のヨナ is 41 volumes and still going (though will have an end at some point)! But I’ve also loved 3 volume manga I’ve read. Sometimes 8 feels like too much, and 12 feels like too little… All depends on the story.