These guys have 6 years of battery data on a range of electric cars. Each model is different in terms of degradation, but it seems that over six years time you lose around 12% of your battery capacity. This means that if your car was able to drive, say 523 km (Tesla Model X), after 6 years you can expect it to have a range of 460km. So long as the graph continues, after 12 years you have a 397km range.
Source: Geotab – EV Battery Degradation