I have the full-blown PFTs done on a yearly basis. This includes the test where you do the huff-puff-huff-puff against some resistance, as well as the test where a tiny amount of nitrogen is blown into your lungs.
In order to chart trends in lung function, my PFTs are done with every clinic visit. The minimum number of times this should happen is once each quarter (i.e. four times /year). I've been in clinic so often this year that I've probably already got almost 10 on record.
When inpatient, our respiratory team conducts PFTs twice a week to measure progress made with the course of IV treatment. They also do the 6 minute walk test at this time.
As to the "box" thing, all of my PFTs are done in there. It's the computer program that they run which determines which "type" of test they're doing--full assessment or spirometry only. Results obtained when in the telephone booth (even if the door is open) are more accurate than the others because it eliminates the possibility of "blow by" air disrupting the measurements.