I have yet to play the co-op unfortunately.
It's not short if you play it as intended. Play till you die, restart, play till you die, restart etc...
Lots of stuff to do, people to save, bosses to fight etc... I'm not sure what my play time was the first time I finished it, but I restarted seceral times on purpose to level up some more.
I'm half way through it again trying for a perfect run (missed 2 or 3 survivors the first time).