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.
Age 63. Other than cancer, in good health; BMI 20
Pre-op: No symptoms; PSA 5.7; Gleason 4+5=9; cancer in 4 of 12 cores
7 March 2008, RRP, non nerve sparing
Two nights in hospital; catheter and staples out after 7 days
Continent, no pads needed from the get-go
Post Op: Stage pT2 M- N-; clear margins and lymph nodes; Gleason 4+4=8; prostate weight: 37gm
6-week and 7-month PSAs: 0
Bimix injections working well