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. 2  at least 2 hours in the fridge. 3  10 minuites to half an hour, wait tilll its comletely hard and dry 4  ! If it hasn't set in an hour, it isn't going 5  to. Couple of hours should do it if you haven't made it too runny.