How often do you monitor pituitary adenomas once established as non-functioning?  

That depends on the size and location (close to the chiasma?). Without pathology, which may help determine aggressiveness (mitotic rate, Ki67), it is impossible to predict the growth rate. For macros, I repeat an MRI in 6 months, and, if no change, I double the interval. For micros, there is recent ...

It depends on the size, but in general, annually.