A paper I came across recently said that of the men who encounter BCR (biochemical recurrence -- a return of a confirmed PSA of 0.2 or higher and rising) two-thirds of them have it within the first 2 years after surgery. After that the chances decrease as time goes by. A 2016 study found that the chances of BCR are quite small if your PSA remains <0.04 at the 3-year mark, even for those who had a positive surgical margin.
Still, some men do encounter a rise after 10 or 15 years, so while some studies have proposed you can stop testing after x years, it's probably wisest to continue.
Which test you use is probably less important than using the same test at the same lab, but for G6 (3+3) men with no adverse features found at RP, as long as the PSA is undetectable you should be good to go
