How many long h-mangas are out there anyways? Most of them have enough chapters to cover 3 volumes at most.
You have some like certain Toshio Maeda's works (only because I've read just now that he is coming to Lucca 2014 so I remembered about him) that span 5/6 volumes.
Otherwise I can only think of a few like Haruki's Parasite Doctor Suzune which, while a bit more mainstream, is 6 volumes and still going, or Ring x Mama which is 5 volumes long.