Minimum depths for sand cement screed floating on any tye of insulation in domestic settings is 65mm. Minimum 75mm for commercial. It should also be reinforced with either d49 steel mesh or polypropylene fibres. There is no recommendation for maximum depth but this will be governed by available height, cost and drying time. As it is very difficult to get sand cement under the pipes with UFH the minimum depth should be measured from the of of the pipe so most heated sand cement screeds will be 75mm to 80mm.
If it is a Gypsol screed you only need 25mm cover to the top of the pipes so the screed will be much thinner. and a Gyvlon screed will need at least 30mm cover to pipes. Typically anhydrite will be 45 to 55mm deep with UFH. No reinforcement is needed for anhydrite screeds either.