As far as how often? I'd say that October is hit or miss, but if your driving in the mountains, and snow is down or the roads are getting slick, you at least want to have M+S rated tires.
http://www.colorado.gov/cs/Satellite?c=Page&childpagename=StatePatrol-Main%2FCBONLayout&cid=1251594537897&pagename=CBONWrapper
http://www.tirechain.com/COLORADO-ROAD-CONDITIONS.HTM