Boulder County defines a short-term rental as any rental period shorter than 30 days.
Boulder County Land Use requirements for short-term rentals and licensing:
https://www.bouldercounty.org/property-and-land/land-use/planning/land-use-code/
Short-term rentals must be registered and approved by the County. For inquiries, contact Boulder County Land Use Department:
https://www.bouldercounty.org/property-and-land/land-use/planning/ask-a-planner