How many days should I wait before back filling?

Just today a friend of mine replaced an old busted retaining wall that's only about 30" high by 60" long. He built it with four courses of 8"x 8"x 16 cinder/concrete blocks. He also filled the block voids with cement. How many day should I let it set up/dry before putting dirt behind it? There won't be much pressure on it but I want to be sure it's mostly cured before putting dirt back against it. We also put a couple of pieces of 3/8" rebar in the blocks.