Template sensor for "hours since the washer finished"

I want a sensor that shows how many hours ago the washing machine finished, so we stop forgetting wet laundry.

Use last_changed on your washer state sensor:

template:
  - sensor:
      - name: Washer done since
        unit_of_measurement: h
        state: >
          {{ ((now() - states.sensor.washer.last_changed).total_seconds() / 3600) | round(1) }}

Perfect, thanks!