Report Abuse. 1  you don't say what sort of icing it is. if its Royal it will take at least 2 hours or more to set. tanika971 Â· 8 years ago. 2  at least 2 hours in the fridge. themorbidhippie Â· 8 years ago. 3  10 minuites to half an hour, wait tilll its comletely hard and dry ! DONT START AGAIN! Iggy POP Â· 8 years ago.