It appears there's a maximum extension limit for the portals. I spawned a greater carnage, then popped 10 green carnage beacons to extend it on each of my 5 man team. The portal stayed up the same amount of time as when I just used 1 green carnage on each. Also I tried spreading out the extensions, the portal actually disappeared without spawning any more right after one of my beacons was used. Would be nice if blizzard would provide some kind of concrete information on how this stuff works, a progress bar or something would be nice.