How long do you think it should take to play through a demo?

1-10 minutes
11-20 minutes
21-30 minutes
Other (please specify!)

Depends on what you're trying to show off, I suppose. Content is more important than how long it is. You could have a 10 minute demo that sucks and a 2 minute demo that kicks ass. So, if you're making an RPG, just find a good start and stop point that gives a general teaser / summation of what your game is about.

For a medium one around 7-8 minutes, then?

I don't have any fancy scripts becuase I'm still in the process of learning ruby (In other words I can't be bothered to learn it right now) and I dis-like having a spoiler full of credit which would probably deter people. I used loads of events though. If have used more, but at like the sixth option in chat I start getting horribly confused.

For a medium one around 7-8 minutes, then?

Just play the game from the beginning and get to the point where you think it would be good to end the demo so that the player want to get the full game. Like after something has happened or if the main characters sister have gone kidnapped or something like that.

Try making a Demo get to a Point of Awesomness in the Game Where people wanna go on as Stripe103 said,but make sure not to much is revealed.When I do a Demo I would usualy take out Features by making an Alternate Version without specifics that may make the Game more fun in the Long run or try to Cut out like Random Dungeons for treasure and Farming Items within to keep it where they wanna try the whole thing.