Assuming you are comming from outside of the US, the Visa requirements to Visit colorado are the same as your that of entering the country in the first place.
A typical visitor's visa gives you approximately 6 months.
If you are comming from a country that does not need a visa to come to the US then your limit is typically 30 days.
The best place to look would be the uscis.gov website for country specific Visa requirements.

That all depends on where you're traveling from. Check the U.S. Department of State's website for information on visa requirements (http://travel.state.gov/visa/temp/types/types_1265.html).