Almost Immediately after the payload is deployed, the second stage does a deorbit burn. So maybe an hour? Each Shuttle launch ended with ditching the enormous e

Almost Immediately after the payload is deployed, the second stage does a deorbit burn. So maybe an hour? Each Shuttle launch ended with ditching the enormous external tank into like the Indian Ocean, far more dry mass in the atmosphere/ocean than a Falcon 9 upper stage. (Also, the ET was about $100-150 million apiece.)