Do you want to dry camp and/or boondock?
What are your interests? State parks, National Parks, cities, historic sites, museums, hiking, sightseeing by auto, etc.
Are you towing a car/truck?
Do you want a quick over view of most of the state, moving every other day, or would you be happy with concentrating on one area of the state, spending 4-6 days in one spot and then moving 100 miles or so and spending another 4-6 days?
When we were working and taking 2 week vacations, we decided on one area we were interested in and then got there as quickly as possible and then spent many days in one spot before moving 50-100 miles.
BTW, September is usually a great time to visit Colorado. Fewer people and usually great weather.