how much hours of game time should be there minimum and is it fine if we release like a demo version of the game here and release the game seperately
I typically aim for an experience that takes 5-10 minutes. That let's you really refine what is fun about your game and polish that up as best you can. One thing I've learned from doing jams for a while, is that most people don't put a bunch of time into the games they play - once they feel they've understood what the game is about, they rate it and move on. I've. Had games where the average play time is 2 minutes and only a couple people even finished it.
So I'd focus on finding the fun and making a short engaging experience rather than trying to create a large amount of content that most people won't play long enough to see.