I'm sure 30" is possible but 20-24" is more the norm. I've kept a few and never had one exceed 24". They have a RTC like personality, but are much more manageable to keep. They require a wide tank though as their whiskers grow huge, and they dont like not having "whisker room".
JohnPTC has one that is easily pushing the 30" mark. His is the first that I have seen or heard of that got that large. I would agree with Bgonz that 24" is more typical.
a 24" marble is gonna have 18-24" whiskers so they really deserve a tank that they can fully spread out in, that equally atleast a 48" wide tank, hopefully more, otherwise they dont seem too happy. great cats but do require alot of room full grown.