They like to test PSA every three months for the first year or two.
I have unilaterally extend mine now to four months, having had seven undetectable results since my op 19 months ago, and will probably go to six-monthly testing towards the end of this year if the results stay the same.
Two friends had surgery around the same time as me by top London surgeons, and both had recurrence soon after and had to have a course of radiotherapy. I feel very lucky.
Cheers, John.