It takes a while because a major organ in your body was damaged and the radiation damage continues even after you finish treatment. Like a sunburn its still smoking in the spots with radiation. But make sure you stay hydrated and get exercise as much as possible because it will help you heal and get better. I am starting the Livestrong program on June 3rd at the YMCA. A free 12 week program.
I also did a pulmonary rehab program( Once I learned the breathing exercises I didn't continue because it was a far distance away from home) ask your Dr. to send you to a Pulmonologist for an assessment and ask them for a prescription to rehab..This is very helpful and can ensure you are treating any possible radiation damage in the lungs with the right therapy.
Otherwise I am told it takes one year from the end of treatment to be fully restored.