There is no fixed rule on the intervals that one should follow to have PSA tests after surgery. However, a PSA is usually taken three weeks after surgery to check for treatment success.
Follow-up tests then are usually done every three months during the first year followed by intervals of six months during the second year, and every two years thereafter.
Your husband has been missing many follow-up PSA tests. I would advise him to get one done for prevention against any recurrence. We see many guys here reporting recurrence after 10 years of a “clean” diagnosis.
Wishing him a continuous healthy status.