A. Because you are looking only at a slab, the question is one of compressive strength. Although there may not be a hard and fast rule, looking at it from a rational point of view can provide a reasonable guide.
Concrete’s strength develops as it cures, and that process is usually acknowledged to begin at the time of initial set. ACI defines initial set as when the strength of the concrete reaches 500 psi, which for standard concrete is generally within a few hours of the initial mixing. By then the surface has been finished but it’s still susceptible to scratching.
The weight of the block wall is distributed over some area of the concrete surface. To simplify the math, assume that each block is 16 inches long and 8 inches wide, with three webs, and a material thickness of 1 inch throughout.
The surface area of one block where it is in contact with the concrete is 50 square inches. Multiplying that surface area times a compressive strength of 500 psi shows that it could support 25,000 pounds, even at the beginning of the curing period, on the footprint of one block.
Even so, you would still want to be careful starting that early. If someone applied a high point load by dropping a tool or a block just right, it could mark the young concrete.