Medicare.gov has really stepped up to the plate for people looking for more information about preventative services. I frequently use the www.medicare.gov website to keep current on the various preventative screenings and the timings for tests and such.
There are a number of different preventative tests and screenings that Medicare covers besides Colonoscopies, and they each may have a frequency and a certain required risk factors that people should know about. You can read more of that here: https://www.medicare.gov/Pubs/pdf/10110.pdf
Screening Colonoscopy: Medicare covers this test once every 24 months if you’re at high risk for colorectal cancer. If not, Medicare covers it once every 120 months, or 48 months after a previous flexible sigmoidoscopy.