So, big day, the first demo is officially online! There is a lot of content, and you can still play after the end of the demo. The story part of the demo itself should take you no more than 20 to 30 minutes, but then if you want to complete everything available, all the side quests you can already do, you can play for about 2 hours.
It is available there: http://zone11.xyz/undersiege/www/index.html
Again, since it is playable without download, experience some lag at the very beginning depending on your connection.