Kel'Thuzad becomes active and targetable for phase 2 when the timer is between 8 & 9 seconds left. I have run Naxx multiple times each week and it is the same for each char.

on retail or wrath classic?

EDIT, sorry I fail at reading. retail, is odd and weird. i've shortened that timer so many times and it just keeps getting shorter it seems

Let me know how it feels after my changes.