There are so many different opinions on this, but my general experience has been that you should ensure you do it at least once every two years. I usually use some local Denver Plumbers to check it out for me. Getting the plumbing checked every year is my general aim, though, and that tends to be enough for me if I want to ensure that my plumbing works as it should.
My general advice would be to aim for at least once every two years. You risk something going wrong without the problem being spotted any longer than this. I flooded a kitchen once because I waited 3-4 years to get my plumbing checked; a problem emerged, and I eventually had to pay extra to get an emergency service. Avoid my mistake and go for annual check-ups instead of waiting!