Well, my players and I had a marathon session for the new spire.
The Fight against Kyuss itself ended up taking 3 hours. At one point in time half the party had died (they are lucky to have that philosiphers stone on hand) and the other 2 were desperately trying to keep the wormgod at bay.
Eventually they finished him off just as Kyuss' summoned minions were about to tilt the scales against them for what would likely have been the final time.