When should I mow the lawn for the last time?

The lawn should be mowed until, how it's still growing. The key factor here is the Ground Temperature: If this drops—depending on the type of grass—permanently below 5 to 7 °C, the lawn stops growing.

When that point is reached, it may varies from year to year It depends heavily on the weather conditions at the time, particularly temperature, sunlight, and precipitation. For this reason, there is no set date for the last mowing of the lawn.

Rule of thumb:
As long as the lawn continues to grow, it should be mowed regularly.