The amount of time you stayed undetectable since the prostatectomy would be the amount of time until BCR.

As long as your PSA is < .1 Then it is technically considered undetectable. If you had an undetectable PSA, the first time it was tested after the prostatectomy then it’s considered undetectable as of the prostatectomy.

Some people never reached that point after a prostatectomy so they have BCR immediately.

For me, it took 3 1/2 years before my PSA rose to .2 which was considered BCR.