This depends on the subfloor and its insulation properties.
A cement subfloor will be slower without a thermal barrier, as a percentage of the installed heat is absorbed into the floor (approx. 30% on a concrete floor without insulation).
As a rule of thumb, a warm floor can be attained in less than forty minutes when insulating backer boards are installed.
Insulated screeds and concrete floors will take up to 2 hours and uninsulated concrete floors may take anything up to 5 hours to reach the required floor temperature.
To make the best use of the system, insulation should be installed, either below the subfloor or as a layer of insulating backer board, OR BOTH.
– Updated: January 2020