There's probably a statistical value somewhere, but I won't guess and it's probably a waste of time.
Simply put, there are just too many kinds of manga. Successful Shonen Jump ones run forever, or, well, at least more than a few years; so are a few shoujo classics (some legendary ones last something like more than
three decades and
ongoing!
). Others run on more reasonable lengths, and there are always one-shots. Every story has its length after all.
To use an example from one manga-ka, Ken Akamatsu, his first long-running series
A.I. Love You is 8 volumes, his big hit
Love Hina 14, and his latest one
Negima! have long since crossed 20 and is ongoing with no end in sight.
Rave Master is not short though. Rave Master is 35 volumes.