The only long running manga I can read and stay with so fare have been Aa! Megami-sama! (Oh/Ah! My Goddess!) and Skip Beat! Might be because they are more char-oriented/-driven than "plot" driven.

There's a few other long-ish series I've read/kept with, but I'm more sporadic with them, comparatively. (Partially because releases are harder to come by. La Corda d'Oro is one that comes to mind.)

Frankly, I prefer series that don't run past about 20 volumes. 30 is pushing it. (Mid-teens or less is nice.)

If there is a basic premise and actual plot/story given at the beginning of the series, I want that story ENDED at some point and not dragged out longer than needed. I never got into Bleach, Naruto, or One Piece because of that reason. I dropped Air Gear once it devolved into that scenario (that and the shitty pacing. Five 20-page chapters or so for one char's fight in a 5-char fight scenario is DBZ-grade milking.)

I'm hoping TLR:Darkness has a story plan for the long run and not quite devolve into a never-ending series. ("The Status Quo is all-mighty" be damned.) It's progessing fairly well so far with nice developments/progress in each main chapter. (The extra add to the series so far, too.) Ecchi with plot, yay! (Plot with porn would be Velvet Kiss ^_~ )

EDIT:
Goddammit, GC 16... First they off chars (for good reason), and now we're seeing shades of resurrections *facepalm* This better play out well or I'll be VERY disappointed.

Oh, then we seem to be having Inori going yandere... And apparently Shuu got broken... Again *sigh* Well, at least the story seems to be in its main arc now instead of all the build/set up in the first half.

EDIT #2:
Am I the only one who thinks Shaga Ayame of Ben-To is a clone of Hoshii Miki of [email protected]?