I would think you could place the leaves on top of soil immediately, without waiting. And then water after however many days you feel is appropriate, within about a week.
There is no scarring so speak of with leaf propagations (compared to stem cuttings, at least). The critical juncture is where the broken part of the leaf might come in contact with soil (and especially wet soil), that's where you don't want things going wrong. But given that you are never going to be burying the broken surface during a standard leaf propagation, the only thing to watch would be how soon you water. Sooner than a week is optimistic, around than that point you can resume a regular schedule of deep watering.
I water my leaf propagations like my stem propagations, which works out to when the soil is going dry, about once a week in 4-6 inch azalea-type pots.
In theory every Echeveria can be propagated from leaves. (As opposed to say Dudleyas, of which none can be propagated from leaves.) In reality there are physical limitations which make some plants way way easier than others to propagate this way. Those limitations might have to do with how hard it is to separate an intact leaf (all the way to the base, which is that part that matters) or more likely, how much moisture a leaf may hold (fatter leaves hold more water and give you better odds with leaf propagations).
The plant in the picture (Echeveria elegans?) is on the thin-leaved side (ie. lower odds of leaf propagations working) but you will find there are some plants which are just easier to propagate from offsets. In many of those cases the best approach is to dehead (core) the plant, with the grand ambition of forcing several branches that you can then cut, root, and grow on as new plants.
