I don't have the specific time for this but I can give the information I know.
My raid group was in Ravana EX, we were going to have the DRK use him Living Dead for 1 of the mechanic. After the buff turned from living dead to walking dead. me and the other healer healed him up and after the timer ran out, he still died.

To confirm this, me and the tank went to helix to test this out. The DRK took all his gears off so he died faster from 2 adds. When his hp was low enough, he used Living dead. After his hp hit 1, I spam healed him up to full hp and even had a adlo on him. the 2 adds that was on him also went to me from the healing so he took no further damage. He also was healed up 3 sec at least prior to the buff timer run out. After the full duration, he dropped dead.