BUMP!
So the shadow lasts for about 5 minutes real time. (timed it with a stop watch, the shadow lasted 1minute, 47.92 seconds) However when compared to the summon time of say Persistent blade, it is basically useless. The shadow also seems rather weak, as the description says it scales with the cleric, but a level 7 cleric gets a level 3 shadow- making it even more useless in combat-