The time a fully charged power failure light stays operable during an outage depends on the light's battery capacity. On average, this is about 3 to 4 hours.
Check compatibility before you buy
While Power failure lights work in most standard light fixtures, there are some limitations. Some models may have issues when used with dimmer switches or in certain fixture configurations; it is advisable to check the manufacturer's specifications for compatibility with your specific setup.
Brands include Energizer LED Rechargeable Plug-in Flashlight and Ideal Security Inc. SK636 Emergency Power Failure Light.