depends on a whole lot, depends on how advanced the disease is, depends on how compliant you are, depends on even climate.
As for me, that is not nearly as compliant as i should be, once or twice a year, i could probably go two years or more if i were more compliant but my case is fairly mild too, my pfts are usually high 80s or low 90s
someone else might be completely compliant and never miss any treatments and still go in 2 or 3 times a year, it all depends