i've baked bunches of CZs at a time. i tried to make sure none of them were touching, but other than that it doesn't matter. and 2 hours was the *minimum* time i ever baked mine. 'course i was usually cooking other things at the time -- we're planning on roasting a turkey tomorrow and i was gonna put some stones in a little dish on the bottom of the oven while it was being used anyhow.
i've had a couple ChrisCZs set and they don't seem to get/show dirty any faster than any other sim.