You kind of need to establish where the goalposts ARE, not just where they aren't.
I've made it as clear as can be without devolving into the dreaded "What is an RPG?" death spiral. In my first post, I specifically mentioned Quest for Glory, which is an RPG/adventure game hybrid. This is a strong hint that hybrids don't really count. First-person action-RPG hybrids are held to a different standard (although some can be very long), as are roguelikes which revolve around trial-and-error and starting over from scratch multiple times before reaching the win condition (and can also be extremely long), et cetera.
A better approach would be to look at some of the hallowed classics that are fairly definitively "pure" RPGs, and clearly not hybrids of any kind (not an all-inclusive list): Wasteland; Betrayal at Krondor; Darklands; Planescape: Torment; Arcanum; the various Realms of Arkania titles; Baldur's Gate 1 and 2; Ultima VII; Fallout 1 and 2; and of course Wizardry 7 and 8, which are absurdly long even if you're hustling. We may not be able to pin down exactly what an RPG is, but we can be sure that all of these games are definitely RPGs. Some people would argue that point too, and have on countless occasions, but they can blow me.
Quite a few of the above CAN be completed (without meta-cheating, foreknowledge, or speedrunning) in 20-30 hours, including Fallout, but it's clear that there's a lot more meat on their bones than arrowing directly for the conclusion of the main story. Every single one of them are over 30 hours if you take your time to poke around, and many will clock in at 40-60 hours. This applies very neatly to many RPGs that are technically hybrids as well, such as V:TM: Bloodlines, Daggerfall, Morrowind, and Deus Ex.
I can accept System Shock 2 (hybrid or no) and a single hustling playthrough of Fallout as evidence that, once in a blue moon, an RPG can be short, or at least on the shorter side. However, these are the exceptions that prove the rule. In my mind, a definitively short RPG is one that lasts for 10-20 hours and then is pretty much done, either because it's not particularly replayable or because one has no desire to replay it. Not even SS2 falls into this category, since everyone (everyone who's cool, that is) has played it at least 2-3 times.