My personal opinion only. I am no doctor.
I think the time between MRI's varies between patient and Dr and will depend on a number of issues.
My treating Radiosurgeon recommended a 12 month MRI, but my local ENT specialist was happy to do the next MRI in 2 years.
This was on the basis my AN was small, it had been treated, and probably would not have grown all that much.
People on W&W especially with small tumors could have MRI's every two years.
Radiation in a way is also watch and wait for 5 Years
Your tumour is quite large, which means radiation failure rates are higher, and if your tumour starts to grow again it may start to cause more serious complications quite quickly, as opposed to a small tumour, which could take years. Also at 3 years if radiation has failed its normally the 2-4 year window that problems are detected.
I am sure you could stretch this out for a few months, but at the end of the day the treating Doc would have scheduled the MRI for 12 months because he saw it as being the most appropriate.
Just my thoughts on this
PaulW