Here's an approach from the Cleveland Clinic:
http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/gastroenterology/inflammatory-bowel-disease/
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Cleveland Clinic said...
The frequency with which colonoscopic screenings should be performed varies according to the extent of colitis, duration of disease, and history of primary sclerosing cholangitis. Because the risk of cancer is low throughout the first decade after the diagnosis of ulcerative colitis, surveillance need not be performed more frequently than every 3 years. As the cancer risk increases, the testing interval should shorten. One reasonable approach calls for tests every 3 years for 12 years, then every 2 years for 10 years, and annually thereafter. Patients with primary sclerosing cholangitis have an increased risk of colorectal cancer, so endoscopic surveillance examinations should be performed annually.