Final Fantasy Mystic Quest is the quickest one that I can think of from my own personal experience, but considering that some don't have in-game clocks, I may have played shorter. I consider it a nice little game to casually play through over the course of a weekend. If I remember correctly, it takes me about 12-15 hours.
Chrono Trigger has always taken me about 20-22 hours to do everything, even on my first time through.
I'm actually one of the few RPG players out there who thinks less is more. Nothing kills an RPG more easily than when it's too long for its own good. Considering that a lot of RPGs are too easy and repetitive, a long length can just be brutal. Don't get me wrong, I can put hundreds of hours into an RPG if I have fun the entire time (such as with the Star Ocean series), but man... games like Lunar go from being enjoyable to miserable once you pass the 25-30 hour mark.