When needed ... It's like asking how often I have a hood ... As soon as I find the icing layer too big, I turn off the freezer, pull things out (put them in the fridge) and leave the freezer open. I'll wait. After a while, I use a plastic spatula to gently break off the part of the icing that is left on and throw away the ice and water. I'll wait. After another moment again. In the end, I wash the inside with water and detergent, sometimes with vinegar, wipe it dry, stuff it back - and it's done. : o On average, maybe once a year - I use LockLock boxes for food (so they don't freeze) and the seal is fine.