Any rule of thumb for how long it takes for cancer to double in size

I've never heard of any rule of thumb. Your Gleason 6 is a low grade cancer, so the PSA doubling time is likely to be a period of years, perhaps many years. A more aggressive cancer might double in a period of months.

A low grade cancer might turn aggressive, then again it might not -- unfortunately there is no way of knowing in advance. This is where the active surveillance strategy has an advantage -- you monitor the cancer closely, and treat it only once it reaches a certain threshold. This avoids treating a cancer that may never need treatment.

Biopsy readings are subjective, and what looks like cancer to one person might look more normal or look like a different grade to another. 

If you have not already done so, first thing to do is to have your biopsy slides sent for a 2nd reading by an expert pathologist: after that's been done to confirm where you stand, you can consider your options more fully and logically.

Everyone is different, but unfortunately most people including many doctors have thier pet theory about the best treatment option which they'll want to tell you about.

A good  idea is to do your research on the various treatment options: the best comprehensive book I've read on this subject is the publication by Lee Nelson MD, "Prostate Cancer Prevention and Cure". It covers diagnosis, stages, lifestyle changes, diet, supplements, treatment options, selecting your doctor, etc.