Suppose it depends on the length of the total game. Danganronpa V3's demo is an edited version of the 1st chapter that's about 1 1/2 hours long and that game takes 30-40 hours to complete but then again it depends on how your game is structured and how story heavy it is. Technically Deltarune chapter 1 and 2 is a demo (don't know about now but it was classed as a demo on steam) and that was about 5 hours in total. How long is the game's play time? How much story does it have? I think a demo shouldn't be any longer than 2 hours but if you have a long, story heavy game then that's a different situation.