Around 5 volumes for shoujo manga, around 10 volumes for everything. In exceptional cases, 15, but 20+ is overkill in most cases. Of course my favorite manga Hajime no Ippo is pushing 90 volumes, so take what I say with a pinch of salt. 😀

Quote from sarcastigirl

Quote from robbit

endings make me sad, so i say infinite.

Totally agree. 😃

kind of agree.
If a manga is constantly good I'll always want more of it, but in my experience most mangas at some point either involve elements that don't mix well with the previous ones ( berserk (imo) ) or reach new levels of ridiculousness( air gear (imo) ).
So I'd say long but with an end: 20 vol

[img]http://img.userbars.pl/69/13603.gif[/img]

My opinion is this

Comedy: 1-5 Vol
Romance: 5-20 Vol
Adventure 20+ Vol
😁

Whats a Flapple?