A method and apparatus for determining estimated remaining longevity for an implantable stimulator. The device employs pre-calculated numbers of days for various combinations conditions of device usage parameters to determine remaining device longevity based upon identified actual conditions of device usage and employs the determined longevity to change longevity indicator states in the device. While between longevity state changes, the device the identified conditions of device usage and adjusts the determined longevity if the conditions of use change significantly. The indicator states may correspond to one or more of Recommended Replacement Time (RRT), Elective Replacement Indicator (ERI) or End of Service (EOS).