The time it takes for an upright freezer to reach its maximum low temperature depends on several factors, including the initial temperature of the freezer, the ambient temperature of the surrounding environment, the size and insulation of the freezer, and the freezer's cooling capacity.
Generally, modern upright freezers are designed to cool quickly and efficiently, and they can reach their maximum low temperature within a few hours after being turned on for the first time or after a defrosting cycle. However, it's important to note that reaching the maximum low temperature doesn't mean that the freezer is at its optimal operating temperature for food storage.
For optimal food preservation, freezers typically need to be set to a temperature between -18°C to -23°C (-0.4°F to -9.4°F). It may take a bit longer for the freezer to stabilize at this set temperature, especially if it's initially filled with a large amount of unfrozen food. In some cases, it might take up to 24 hours or more for the freezer to reach its desired operating temperature after a significant load of room-temperature items is placed inside.
To ensure that your freezer is operating at the correct temperature, use a freezer thermometer to monitor the internal temperature. This will help you verify that the freezer is functioning correctly and that your frozen food is being kept at a safe temperature for long-term storage.