Mine was done at three months, and then every three months since, except now my last result was extremely low so I've been moved to every six months.

The explanation given to me with radiation is that they look for the trend--three upwards moves of PSA in a row *OR* PSA hitting 2.0 more than the lowest point (for example, if the lowest PSA you get is 1, then a 3 would be cause for concern).

Generally, after radiation, PSA will drop slowly so don't be alarmed by the first post-radiation test. Mine was .4 if I remember correctly, but three months later it was .1 and then three months after that .04 which is when I got put on six month monitoring.