The real answer? Never.
As with most materials, concrete retains a certain amount of water when in equilibrium with the build environment. However, concrete is a particularly wet material. Using ASTM #2170, the equilibrium relative humidity (ERH) test, most flooring manufacturer guidelines require the concrete maximum moisture to be between 70 and 75% ERH before placement of the floor will be warranted. Furthermore, 70 to 75% ERH is usually about 0.70 to 0.75 water activity. That makes concrete one of the wettest of our “dry” building materials.