I've had the pulmonary function tests about every 5 years. And only because there was actually a need for it. Not as a routine thing.

I think the main thing, if your asthma is well controlled, is to have that well documented, and for that, a regular check up with your doc is ok. No need for a yearly test. The reason that it needs to be regularly documented is that insurance companies always like to fall back on 'preexisting condition' to deny you claims. And you need to prove that although you have asthma, a chronic disease, it was well controlled.