Float life refers to the period the battery will last when it’s under ‘float use’ i.e the battery voltage is maintained at approximately 13.5V during usage. For a layman’s understanding, this is the condition the battery is in when connected to a UPS and there is no powercut or there’s generator backup i.e battery is never discharged.
Battery float life for a VRLA battery varies from 3 years to as high as 25 years depending on the design. Lower the design life, lower the cost of the battery and the quality.