As far as I know there is no time limit requirement to challenge the CNA examination in Colorado after you complete a nursing assistant training course, however, once you submit an application for certification by examination, you have two years to take the state exam.
Example: you completed a CNA training course and did not submit an application to test to the state of Colorado. Three years pass by and you decide to challenge the exam. You complete and submit an application to test. From this date on you have two years (twenty-four months) to challenge the exam. If you do not take the test within the two year period, your paperwork is discarded. You must apply again, and submit a new set of exam fees if you wish to challenge the certification test in the future.