X-ray showed hairline crack at the 5th sacral vertebrae after I fell from stairs on my back. How long to heal & what precautions in daily activity?

Generally such Fx's. Require only some pain meds ; heavy lifting, bending of the back, as they will cause more pain. I am sure after 10 days or so when pain is less, you should slowly and gradually resume your regular activities. Make sure you just do not just rest, as leg/chest clots can and does occur, so keep exercising your both lower extremities regularly, about 3-4 times a day. Taking Aspirin daily would help.