In warm weather, a concrete slab can be walked upon the afternoon of a morning pour. A Contractor will typically allow 2-3 days before applying any loads, but will keep back propping in for a few weeks to prevent collapse. It is designed to reach it's designated strength 28 days after pouring. The slab will continue to expand and contract to lessening degrees over the following years.
Concrete does not dry, it cures. The process of cement "hardening" is a chemical reaction. It is desirable to keep the surface of the concrete wet or damp after it initially sets in order to prevent dryout which ends the curing process and limits final strength. Water is actually causing the glue (cement) to bond with the aggregate (stone/sand) creating the strength you're looking for.
However, care should be taken with water content, as too much water can cause unwanted effects. Shrinkage is a main cause of cracking. As concrete hardens and dries it shrinks. This is due to the evaporation of excess mixing water. The wetter or soupier the concrete mix, the greater the shrinkage will be. Concrete slabs can shrink as much as 13mm per 30meters. This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces. Control joints help concrete crack where you want it to. The joints should be of the depth of the slab and no more than 2-3 times (in meters) of the thickness of the concrete (in millimeters). So 100mm concrete should have joints 2-3 meters apart.
For the best result, care should be taken as to how much water is mixed in with the concrete and as to the placement of control joints.
It is just as important that concrete floors are not sealed before air or bleed water from below have had a chance to escape. In fact, we at Flock Image strongly advise that anyone intending to lay a floor from our Hammerflow or STB range, to do so, after the initial 28 days have elapsed.