I'm thinking of running the first part(s) of SKT at a convention, say 8-12 hours of play.
How long might Nightstone be expected to take?
I wouldn't do that. It's a very bad module full of, essentially, random, meaningless encounters that have zero relevance to the story, or even to the reason why the PC's are there, and the Nightstone itself never comes up in the adventure, or in the campaign. Not only that, but it's also very poorly structured and can be incredibly boring for a long stretch of time because of all the completely pointless areas that hold no interest or clues or anything other than empty rooms on a very large map.
It's about as bad an adventure as you can make, short of being Castle Greyhawk.