Persona is a great series, yet it is far from short. Unless you consider 60-80 hours short, however. But please, don't let that deter you from one of the greatest JRPG's out there. That is complete honesty.
Now, a good short RPG would be something like Radiata Stories, the Parasite Eve series (which is an interesting action-RPG), and the original Fallouts. Also the Grandia series aren't terribly long, but start kinda slow, in my opinion. Now, I would say something also like Kingdom Hearts or some of the Final Fantasies, but those can be relegated to ones ability to blow through a game.
However, you're going to be hard pressed to find an RPG that clocks in below 20 or 30 hours, as it takes usually that long to convey the story in a sense that the developers wish. Yet if this is an acceptable timeframe, then you do have wider options. It just heavily varies.