I only waited 6 months before deciding to get treated, for a couple reasons. First of all, I wanted to see if the darned thing was still growing, since I hadn't had a followup MRI since the initial one that led to my diagnosis. When the first followup MRI showed my tumor had grown about 50% in volume in only 5-1/2 months and had gone from 1.9 cm to 2.1 cm along the oblique transverse axis of measurement, I was pretty crushed to realize it was racing for the finish line. Had it stayed the same size, I would've been content to watch and wait indefinitely. But it was growing, and growing rapidly.
Still, I was somewhat unresolved about seeking treatment until reason #2 for seeking treatment was related to me by my doctor, Dr. Chang (Stanford U. Med. Cntr): it was now touching my brainstem, whereas the initial MRI showed it had been a couple millimeters away from the brainstem 5-1/2 months prior. That was it for me. I didn't want my brainstem to be any more in the way of the radiation beams directed at the tumor (by virtue of the tumor growing more and indenting the brainstem) than it already was at that point, so I decided not to wait. Also, although the next six months of watching and waiting wouldn't likely see the tumor grow to over 3 cm, there was a very remote possibility this could happen and I didn't want to take the chance I would be eliminated as a candidate for radiosurgery (CK in my case).
After a couple days of soul-searching and waffling, I knew what I had to do. From what I've read on this board, most people eventually do get a gut instinct of when to get treated and what form of treatment feels best for them. For me, continued growth and involvement with my brainstem were the final impetuses for me making the commitment to get treated.
I hope that helps,
Tumbleweed