I choose 21+ mostly because what I like is fairly lengthy. But "(I love it so much that I don't want it to stop!)" is definitely wrong. The right options should be:

"It should be as long as needed. No more, but no less than that."

Take Hajime no Ippo for example. It's 70+ vols but I really dont feel it's long. Mostly because the timeflow of that manga is quite slow. The number of box fights is countable (20- if we count all the extra fights?), the number of characters is definite and they also are distinctive.

One Piece? Naruto? Prince of Tennis? Please... the damn things just drag on. I drop them on vol 15 take or give a few. They got no progress worth talking about. Same deal with Dragon Balls. I havent read Tenshi or Sailor Moon so I dont know how those manga are.

Midori no Hibi is an example of short manga that could have been shorter, or longer depended on your taste. 8 vols of hilarious situations of a guy with his girlfriend who is his left hand... Go on! Laugh! Get it out of your system! But the manga is good. It finished when the time is right, which is when some feel it could go on a few more.

Same thing happen with Hikaru no Go. Some feel it should have ended at 2/3, which just go to show not many really remember the name of the manga (Hikaru's Go, not Sai's Go). It go beyond that point 6 vols, enough for 2-3 more battles and Hikaru's first serious international experience - his first deadly battle. Personally I wish it could go on more, but that is the sign that the manga ended well.

Generally, a majority of manga readers is leery of lengthy manga for many reasons (intimidating? short attention span?...) But , compared to other media, such as novels, manga is simply short/brief. Bah! Compare to the monster of 12 vols of The Wheel of Time, no manga has the equal length. (~6000 pages of text)

Let's face it, anything great lasts the span of time. Which to me means it lasts longer than most. I'm not saying that ALL manga that spans over 21 volumes are GREAT, but it must be doing something right.

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Money does matter!
The major dragging factor to buy one that has lost energy in the middle.

As a collector we'd really like to buy a full set but the lengthy repetitive series like Conan are such headaches. As we loved the characters, we enjoyed some good scenarios but the author does not seems like to end it anywhere soon after 60 volumes ...

no, there are some cold animes that will never be subbed but could be good ~

Manga are good as they can be taken out. But now even animes can be watched with portable media players.
The graphics used to be better in many manga but now the anime production has improved a lot.
The match is getting closer.

Don't forget anime = a lot more works.
The number of pictures... every second counts.
Great scenarios can be backed up with marvellous music.
And the professional seiyuu's acting is most likely 10 times better than my own boring reading version inside my mind. And as far as you learnt Japanese, you can enjoy them fully.

So if I know an manga will be produced, most likely I would stop reading but wait.

Any length~
Volumes is not a problem if you are rich and have lot of time...
Volumes is not a problem if the manga is going on with superior quality
Volumes is a problem if the works used to be good but pitfalls going on and is not going to end soon.
Volumes is a problem if its great but ending too soon.