Section 22 of Article V of the Colorado constitution says that “no bill shall become a law except by a vote of the majority of all members elected to each house taken on two separate days in each house…” The requirement that a majority of all members of each house must vote for a bill on two separate days means that bills cannot be heard on second and third reading on the same day. A bill can be introduced, read on first reading, heard in committee, and then passed on second reading in the same (busy) day. This means it takes a minimum of two days for a bill to pass either the House or the Senate.
This two-day minimum per house means that the minimum number of days for a bill to pass is three. A bill could be introduced in the House of Representatives on Day 1, and then sent to committee and to the floor for second reading later that day. However, the state constitution requires that third reading take place on Day 2. After the bill has passed the House, it may be introduced and sent to committee and then on to second reading in the Senate on Day 2. Final passage of the bill by the Senate on third reading and consideration of concurrence or appointment of a conference committee, if necessary, would have to wait until Day 3.
So, the answer is a minimum of three days.