Yes! Demo should give a glance of your work. It should not be too lengthy or contain too many levels. A clean and short demo is a perfect option as audience tells you about what they like and dislike in your game so that you could fix those things. Demo is just like the foundation of the game, if foundation is not good then the building  collapses!!!! As you have mentioned, the demo should be perfectly fit in between A and B.

btw thanks for reading this article!!!!

The main thing to keep in mind with RPGs is that they are huge time investments and the average player understands that it can take up to an hour or more before the game really starts to get interesting. That's personally why I very rarely play Indie RPGs -- there are so many of them and most of them look more or less the same. The ones I have played had very excellent character art and backgrounds. But while the art is crucial to getting people to look at your game in the first place, it's the story that will keep the player there for a long duration.

In terms of demo duration, I think an RPG should be maybe 10% of your whole content. I recently played a point-and-click adventure where 25% of the game was free to play. By the time I got to that 25% I was already hooked, so I went and bought the game to play the rest of it.

That being said, it is also true that the longer your demo is, the greater chance there is of having bugs that might turn away players. However, the mechanics of an RPG are usually pretty simple. Issues like clipping and poor hit detection don't really apply, especially if the combat is turn-based. As long as the story is fairly linear and events play in the correct order, there really isn't much to worry about.