I would request every three month cultures. I think that 6 months is way to long and too many things can happen in that amount of time. When my nephew was little, they only did one time a year cultures, and I think that is part of the reason why his disease has progressed much quicker than the average 16 year old. Things were not caught quick enough and then they couldn't get rid of the bugs that he had.
As for the Tobi...my younger daughter Hannah, has cultured PA, and she is only on Tobi as she has a positive culture, not every other month. That is probably because I allowed for her to go into the EPIC study which is a PA study, and it takes different routes, one way being on one month/off one month or culture basis. She obviously fell culture basis. That was a year and a three months ago, and I think she is almost done with the study, possibly in the beginning of the year. She has cultured PA three times, the first time she took two rounds, back to back and then again six months later. Her PA is still non muciod when it does pop up. I am curious as to how he will treat it after the study is over.