How often should a man in his late 50's have a colonoscopy? i'm prescribed omeprazole as the first one found one benign polyp deemed adenomatous

Depends on risks: For those at low risk for colorectal polyps/cancer (no personal or family history of polyps/cancer, no other risks), every ten years if no adenomatous polyps are found. If polyps are found, the interval is shorter: every five years is usual unless polyps are large or multiple. Btw, Omeprazole inhibited polyps in rats; the same has not been found in humans, but Omeprazole can cause gastric polyps!

3-5 yr: If you have polyp, you should repeat coloniscopy in 3-5 yr.