Glad to hear from you, aquamarmoset!
Here's the best guess we have at the moment:
1) Getting to the end of the part currently available to pre-orderers should take you 2-3 hours, or possibly more depending on your playing style, and e.g. the level of difficulty - easy being somewhat faster, as you'd probably guess. If you like to play like I would, and try to find the fun little details in the game, it'll take longer. You might for example notice a pair of idle skeletons having a chat, if you pay attention. (The in-game time meter isn't quite accurate.)
2) The whole campaign will of course be notably longer than the current version, but we can't name a reliable estimate yet. BUT: We do aim at making the whole game good & interesting throughout, and not just adding hours of boring & random stuff to get the game to last 50 hours.
AND as a nice bonus, we'll also implement a mod sharing system, for all the diverse mods people have already started experimenting with, when using our editor. All the mods will be available to all orderers (if that is the wish of the mod creator).