The American Cancer Society advises at least 30 minutes of physical activity 5 days a week for better health. Engaging in 45 to 60 minutes of "moderate or vigorous activity" on 5 or more days of the week is most effective for lowering the risk of colorectal cancer.
However, any amount of physical activity is preferable to none at all. Even moderate or low intensity exercise or activities such as a daily walk may lower the risk of colorectal cancer. Conversely, increased time being sedentary appears to increase the risk of colorectal cancer.
American Cancer Society. "Can Colorectal Cancer Be Prevented?" American Cancer Society, Inc. 17 Jan 2013. 22 Jan 2013.
Howard RA, Freedman DM, Park Y, Hollenbeck A, Schatzkin A, Leitzmann MF. "Physical activity, sedentary behavior, and the risk of colon and rectal cancer in the NIH-AARP Diet and Health Study." Cancer Causes Control Nov 2008. 22 Jan 2013.