How long does concrete need to cure before applying self leveling compound?
You still need to follow the instructions for the SLC - most (IIRC) say 30 days minimum cure (for the underlying concrete.)
Be sure to keep it damp so it can actually cure. If it dries out, it stops curing, and that's a very likely thing with a thin pour over an existing slab that will want to suck water out of it.