I would say it lasts for a number a rounds equal to the edimmu's HD, so 5 rounds. I don't think fear effects stack, unless it says so in the power's description.
Fear effects stack, last I checked. It's why the Cairn of Screaming Ghosts is so dangerous, stacking fear effects to cause players to jump off the pedestals.
My players are at the first underground level of the House of the Beast and we are playing in 90 minutes. I am excited about them reaching the pit of Screaming Ghosts today ! :P
Reading the ability's description again, it seems it functions like an aura not like an activated ability.