I get xrays about every 3 months or so, maybe ever 6. I get ct scans at least yearly, but I have M.A. and they monitor its progressiveness with those.
I also had some pneumo issues so i had like 10 xrays in a weeks time, and then another 4 two weeks later...
xrays help show the scarring a bit, and ive heard most clinics like to do them at least yearly so they always have something to compare the progression of the disease to each year I believe.