In patients with Stage I seminoma who elect surveillance, how long do you recommend that tumor markers be followed?

I agree with @Dr. First Last that the data is vague. I would add the following: in series where patients are regularly re-staged with CT scans, serum tumor markers add essentially nothing. So if one follows the Princess Margaret Hospital schedule and gets scans every four months for the first 2 to 3...

Data around this is very fuzzy. After 5 years, the recurrence rate is probably about 1% or so, and it would be difficult to justify any testing. Usually after 5 years, I offer that the patient can be remanded to their primary care doctor. I offer to see them once a year thereafter if they desire.