UFH - concrete slab above or below?

[shambolic]

Yes I spotted the fire, so the flue obvisouly goes up one side of the stricture. So what's the slab thickness to give you a 1 to 2 hour heat up time, as this is where the thread sort of started from.

Don't know it has slab, insulation, heating pipes then screed.
It works so never worried that much. I'd need to dig out engineers drawings to see specs.