I can't wait for the jam to start, as I'm a long time adventure game enthusiast and I just needed a reason to get back to designing adventure games.

However, I'm alone and 14 days aren't much (I have some assets and that would speed up things, fortunately), so I doubt I can come up with something too complicated. Maybe a "part 1" to a longer game.

So... what would be in your opinion an acceptable game length?

As they say, it should be the length of a skirt: long enough to cover the subject, but short enough to keep things interesting :-)

But seriously, there is really no right length. It depends on what you think you can get done in 14 days. My advice is to always plan a shorter game than you think you should because time often goes faster than you expect. Longer is by no means better, as long as it's not SO short that there's almost nothing there.