Hey @gently
I'm with @edmond1971 here. It's not so much a time thing as an indicator. If the PSA starts to go up, I'd probably start doing research on where would the PSA level have to be to be likely to give me the most accurate results.

If the metastasis is at the micro level, the PSMA scan might miss it. (which may be what happened with my first scan given my history)

I had the scan after my PSA went from .039 to .091 in 3 months. It was negative although there was obviously something going on. The cancer was probably still too small to be seen. There has been some research on this topic and one of the other members here has shared some links and charts regarding this. @kujhawk1978 I think has done a lot of research on this topic.

I then had salvage radiation and ADT for two years. (just got off that in June "knocks wood")

My timeline/history during that time was:
09/02/21 Radical Prostatectomy -
10/22/21 PSA Result of < 0.1
01/26/22 PSA Result of 0.039
04/26/22 PSA Result of 0.091
05/20/22 PET CT PSMA for Prostate Negative results for metastasis

Best of luck to you!