A freezer will stay frozen for from 4 hours to 36 hours without power, it really does vary that much.
The shortest holding time would be for a small front opening unit that is not full, and in warm suroundings.
The longest time would be a large, fully stocked, top opening freezer in a cold place.
And remember that meat, for example is still safe to eat for at least a couple of days after it defrosts.
Most freezers have a "fast freeze" setting, this reduces the temperature below that normally achieved. Use of this control before the power cut will reduce the internal temperature and therefore increase the holding time.

Refrigerators wont normally stay cold for as long.
Perishables should be consumed promptly or discarded if the power is off for more than a few hours, unless of course an unheated area is at a similar temperature to the fridge.

A reasonable sized generator should be able to run a fridge and a freezer.
As the fridge needs near continous power, it might be best to only plan on useing the fridge for perhaps the first 2 days of a long outage.

If preparing in depth for long term power outages it would be worth considering ultra high efficiency freezers and fridges.
They use little power and can be battery powered, with the batteries charged by limited generator running, or from PV modules.
My fridge uses less than 1KWH a week !
That would be a week from a single deep cycle battery, or months from a limited fuel supply used to run a generator for perhaps an hour a day.