WillieT: if 28 days is the ideal time before laying block, what would you consider the minimum time since many people don't have that much time?
A week would be the minimum in my estimation. Two weeks would get you well into the curing cycle.
Google "Concrete Curing Cycle", and you should find graphs showing the strengths achieved for a specific period of time.
I have truly seen, with my own eyes, 1/2" deep indentations sunk into house slabs by placing a cube of blocks upon the slab within 3 or 4 days of pouring it. One can only imagine the future damage that must have set in motion.
"True eloquence consists in saying all that is necessary, and only that which is."
FranÁois Duc de La Rochefoucauld