I am so sorry to hear about your friend. I've worked in a Neurosurgery ICU and frequently cared for patients who were in "induced coma." Basically, it is just a sedative.....actually think of it more as an anesthesia-type drug. I would be willing to guess that the drug they are using is called Diprovan (propofol) which is white and comes in little glass bottles that hang as an IV. The IV is titrated to keep her at the desired sedation level.
These medications are a short acting drug. The reasons for this is so that the patient can be "woken up" quickly so that her neuro status can be assessed. Depending on the physician, usually every morning on rounds, and sometimes once per shift a patient is "woken up" by temporarily turning off the medication. When the patient becomes aroused, she will be asked questions to see if she can respond, her reflexes will also be checked at this time. After that is evaluated, the medication is turned back on and again, the patient will quickly go back into the "drug-induced coma."
The time varies how long each person will need to stay in the "coma" depending the the severity of the head injury. Her intracranial pressure (ICP) will most likely be monitored. The ICP does exactly what it indicates.....it measures the pressure in the brain through a catheter in her head. It is connected to a transducer which gives a number value. (Normal is between 4 - 15 mm/Hg OR between 50 - 200 mm/ H20 depending on whether is a mercury or water set-up.) Sometimes, if the pressure becomes too high, the ICP can be drained to relieve the pressure.
Her ICP is going to be an important number to watch for to evaluate if she is ready to be "woken up" from the "coma." Something that may give you some peace of mind.....these types of drugs cause the patient to have NO recollection while "under." Even during the brief periods where the patient is temporarily woken up to evaluate their neuro status, patients rarely remember those brief events. So it's good that she won't have to remember this traumatic ordeal. This will keep her relaxed and help bring that ICP pressure down.
You asked "how long" a person can stay in this type of state. The answer to that is.....as short as possible. It's not so much the "drug-induced coma" that hurts her....it's the fact that the longer she has the brain swelling, the more chance there is for some type of damage to the brain cells. So the quicker that pressure can go down....the better.