The best time to use Shadowfiend on Lurker is on one of the ranged adds. Call for the other DPS to lay off of it a bit and you'll get a full 15 seconds, guaranteed.
I never have any trouble using my fiend on Lurker himself as long as I pay attention to my timers for his Spout/Submerge abilities. Just use it right after Lurker completes a Whirl and isn't close to starting a Spout or Submerge. However, using it on one of the adds is definitely a more sure fire time for a full duration.