15 hours would be an ideal time-frame for a decent adventure...
If an adventure game gets beyond that, it usually could have done with some tighter "editing"... (kinda like The Longest Journey - brilliant, but I couldn't help feeling it could have done with trimming one or two scenes)
Whenever a game extends beyond the 15-hour kind of ballpark, its inevitably making up the time with time-filler gameplay.
GTA does it, the Final Fantasy games do it... they even have a decent amount of story, but when you really think about it, 90% of the gameplay is just stretching out the time between "events"... and thus, pretty eventless. (Not to say this isn't a valid idea - I enjoyed the GTAs and FFs a lot)
I think tightly scripted adventure games are always going to have a different price-to-length ratio... because so much more work goes into the same time frame. That said, in the same way that a 12-hour long linear action shooter is hard to swallow, so is 5 hours of adventure game for the same money...