Garage freezers in the Valley live a hard life. Ambient heat, dust on the condenser, and a door that gets opened far less often than the one in the kitchen.
A chest or upright freezer that has drifted warm is usually one of three things. The condenser is choked with dust and cannot dump heat, which in a Chatsworth or West Hills garage in August is close to inevitable. The evaporator fan on a frost-free unit has stopped. Or the defrost circuit has failed and the coil is behind a wall of ice.
The opposite complaint, everything welded into a block of frost, usually means the door gasket has stopped sealing or the lid is being held slightly open by whatever got stacked at the front. Warm humid air gets in, and it condenses on the coldest surface it can find.

Often not. Vacuum the condenser coils twice a year and give the unit clearance on all sides. Many garage freezers are running at the edge of their rated ambient temperature and a clean condenser is the whole difference.
Roughly 48 hours in a full freezer with the door kept shut, about 24 in a half-full one. Do not open it to check.