There is major confusion about when power will run out and when the ship will crash. The ship can continue in a normal, non-decaying orbit as long as it has power to make course adjustments. Once the power is out the orbit will start to decay. Likely the Enterprise had another 2-3 days before it would crash. However, life support would fail before the ship crashed, but there would be enough breathable atmosphere on the ship to last at least for many hours if not a day or more.