Medicare is quite straight-forward in their coverage for colonoscopies:
For screening colonoscopies, Medicare will pay for one screening colonoscopy every 120 months (10 years) if you are considered low-risk. This means you have:
If you are considered a high-risk screening, meaning you and/or a first-degree relative have a history of colon cancer or colon polyps, Medicare will pay for one screening colonoscopy every 24 months (two years).
If you have symptoms of colon cancer, Medicare requires a co-insurance or co-pay for a diagnostic colonoscopy.
Recently, under the Affordable Care Act, Medicare has amended its coverage for screening colonoscopies to waive the deductible and co-insurance for anesthesia.