wartable quest

Was there any change to your system time? Perhaps in your time away your console's time and date was reset? Updating this should help the mission to complete, and free up Cullen. 

I found that my Xbox 360 would no longer hold the system date after a while and would keep resetting to something like Jan 1 2004. If this is happening to you, that is why your remaining time is so high. The war table missions time stamp the date you started and compare against the current system date to see if enough time has passed to complete. So you can manipulate this mechanic by setting your system time forward and immediately going to the war table to complete long missions.

But it also works the other way and if your system time goes backwards before completion, it adds to the time left to finish the mission.