Once you culture it, you will probably culture it forever. Unless you are a young child and you're culturing it for the 1st time, or you had it once before, it went away, and now it's back again.
I'm 24 and I think most CFers my age culture it 100% of the time. So by no means do I treat it all the time.
But it has its ways of creeping up. So TOBI or Colistin are good ways of keeping PA levels down so that you don't get an exacerbation.
IV antibiotics are another method to treat a flair up. Tobramycin is a commonly used antibiotic.
Quinolones, naimly Cipro, are effective for some mutations of PA. That's nice because it's a pill and you don't have to be in the hospital.
Hope this helps.