How long should a demo be? Depends. A lot.
What purpose has the demo? What is the target audience? And since you post this here on Itch and you do not seem to be an established developer, nor have a finished game, what is the plan for publishing the game?
Why do people play demos? I would say, to try before buying. For games that were successfully advertised to them (or which they found by accident).
They are needed on Itch because there is no easy refund policy and because Itch does not advertise the game for you. And people are not gonna buy an unknown game from an unknown developer just like that.
But they also might not be the best choice of strategy for some games. The Itch crowd is used to pay what you want games and free stuff. If you only release on Itch, pay what you want is said to often result in more sales, but milage might vary greatly here. So instead of doing paid+demo, free+optional paid content might be the better strategy.
My suggestion, also with the genre in mind, would be a public/free version of the game, instead of a demo version. And a paid deluxe version with more bells and whistles or whatever seems suitable. A web version might do the trick. If you want to see a very successful implementation of this, Backpack Hero comes to mind. Their public version is an earlier version of the game as a web build. They do not have a "demo" version of the game.
So. How long should a demo be, if a demo is what you wanna do.
For a roguelike game, do not limit it in "length". If you make a puzzle game, or some adventure maybe, you might limit the level / scenario you can reach. But roguelikes have that repeating dungeon dive mechanic. Players will expect to replay the game, not reach more difficult puzzles or later parts of the story. So a demo should give them a experience they want to repeat and replay. And in my opinion, it will be very hard to give that experience by limiting the "length" of the demo. Imagine playing Slay the Spire to the first boss as a demo vs. limiting the playable character. Which demo would work better? We will never know, because it has no demo that I would know of, as it was released in a way that can get away without having a demo.