at least half a day is usually safe. All Appliances sold in EU have this in their data sheet: ‘power cut safe “X” h’ defined as ‘temperature rise time’

Me example from last year: 4h at about 20C Ambient temperture: 4 degree Celsius loss from -20C

Too bad smart home temperature logging is failing too while power is off: https://twitter.com/mxfh/status/1194646175644897281/