A freezer that requires manual defrosting should be defrosted any time it accumulates one half-inch of ice on any part of the interior, but some freezers are self-defrosting and do not require manual defrosting. The operation manual that accompanies the appliance should list any maintenance it needs.
There are several methods for defrosting a freezer. Turning the freezer off and waiting until the ice melts is the safest method, but a hair dryer or fan can also be used to hasten the process. Putting bowls of boiling water inside the freezer can also help. Another method is to use a hot cloth to help break down the ice.