Most of the missions with casual friends is approx. 1.5 hours or so.

~D

My group is around 2.5 hours average to finish a mission. I generally play against 2 Rebels playing 4 heroes, so the early missions are actually 1-1.5 hours, but the later ones are quite longer as it takes them more time to go through abilities on each character. At that point they have so many tricks it is hard to remember everything. This next campaign we are trying with more people, I expect the missions to still take as long as there will be more chatter/deliberation, but I also expect it to be a generally better experience for the Rebels as there is less ability management.