Milly is selling girl scout cookies.She sells 8 boxes every 3 days. How many days will it take her to sell 56 boxes?

Milly sells 8 boxes every 3 days, so in 1 day she sells 8/3 = <<8/3=2.6666666666666665>>2.67 boxes.

To sell 56 boxes, it will take her 56/2.67 = <<56/2.67=20.9375>>20.94 days. Answer: \boxed{20.94}.