Yep, keep it moist/use plastic on top to control the dry out process. If it is not structural, IE: the concrete is just setting there like a barrel full of sand and does not provide any of the support, then 24-48 hours sounds OK. If it is part of the structure, such as a crossbar passes thru the concrete, or the concrete keeps the sides of the barrel from buckleing, then I believe 28 days is the magic number to achieve the lions share of the concrete structural integrity.