My games are mostly RPGs and so 8-10 hours long. My demo is simply the first hour of the game. You play the game normally for an hour and if you like it, then you can buy the whole game. If you don't have the time, there is a short gameplay video of a key scene in the game. The demo is free, obviously, but I'm always surprised that few people load it down. Why not? What better way to test a game? For those interested: https://criacuervos.itch.io
Viewing post in Why demo is an Important aspect of the game?
you can cut it down to like 20-30 minutes cause if anyone likes your game they'll play for the whole duration but if they encounter any issue with that they'll surely leave that. Keep the key features in that 15-20 minutes session of the starting to make an impact and the rest would be the normal gameplay!!!!
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.
He(@40wattstudio) is absolutely right the demo should contain 15-25% of the game with hook of the game that may be an artwork, a good story, ,great actions, soothing environment, great puzzles etc. And that would raise the interest of the players to buy the full game!!!!