Fire & Fury is a pretty fast set of rules, but some of the ACW battles are big (and many of the scenarios in the scenario books are big).
Just as a for instance, the Shiloh scenario is good, but requires a lot of troops and probably 5 or 6 hours to play.
I halved all the troop numbers and managed to play through Shiloh in 3 hours.
So if you stick to 8-12 brigades a side, you'll finish a game in 2 or 3 hours.
If you insist on refighting big battles, you'll be there all day.