How long can a person can be hooked-up to a ventilator in I.C.U.?

If a person is on a ventilator with a tracheostomy done and has developed ventilator-associated pneumonia along with other complications such as blood loss,blood pressure drop etc. and is in a semi-conscious state. The patient is lying stable but in a crippled condition with no signs of recovery. What are the chances of his/her improvement? "to what end" is it possible to keep the patient dependent on the ventilator?