Ideally you should never stop taking your Advair unless advised to do so by your physician.  Asthma can be a tricky disease by not showing symptoms for days, weeks and even months.  When you do have asthma symptoms, and you are on your Advair, your symptoms will be less severe and easier to treat. 

As far as a pulmonary function test  (PFT) is concerned, your doctor will let you know if you should stop taking any medicines before the test. Not only can a PFT diagnose asthma, it can also show your doctor how well your current med regime is working.  

If you have further questions about your need to take Advair, or what to do prior to your PFT, you should call your physician.