IMO (and my doc's), yes, keep testing -- it's just a blood test added on to your others so no real inconvenience
. PCa can recur even after 20 or 25 years, even if, especially with a G6 and no adverse features on RP, this is highly unlikely. (Whether this is, e.g., from long-dormant or slow-growing micrometastases, or newly formed lesions in remaining healthy tissue is, AFAIK, not known.)
However, here are two studies that reached a different conclusion: Determining When to Stop Prostate Specific Antigen Monitoring after Radical Prostatectomy: the Role of Ultrasensitive Prostate Specific Antigen
This long-term review indicates that if patients have continuously undetectable prostate specific antigen levels by an ultrasensitive assay for 5 years, prostate specific antigen monitoring can be stopped with an extremely low risk of subsequent biochemical recurrence."
(Note that an ultrasensitive PSA test is one that reports to 2 or 3 decimal places.) Prostate Specific Antigen Testing after Radical Prostatectomy-Can We Stop at 20 Years?
Men with delayed biochemical recurrence have favorable clinical features and improved survival. Men with undetectable prostate specific antigen 20 years after radical prostatectomy had a low rate of recurrence and no deaths from prostate cancer. This suggests that 20 years is a reasonable time to discontinue prostate specific antigen testing."