When I do a foundation, we let it cure a minimum of 7 days before backfill. We also install the first floor deck to hold them together. With a garage wall and before installing the slab and back-fill we wait 14 days...ewww.
Then we back-fill in what is called lifts and keep heavy machinery away from the walls. The lifts are done 8 inches at a time and compacted for a garage floor. We also wet it slightly as we work.
Most folks go ahead and fill 3-4 days after, and just fill the hole then compact. Sometimes it works out...and sometimes there are really bad results. Concrete actually takes many, many years to cure...the Hover dam is still curing.