There are no special laws dictating when you can receive a bill for services. Delays in medical billing are common because the hospital first works with your insurance company to determine what services were covered by your policy.
Mr. McReynolds is correct.
That said, there are time limitations within which the hospital must sue you for payment if it going to do so. That is called the "statute of limitations" and will be found in CO Statutes. The basis of the cause of of action will define the applicable statute of limitations. In the case of a medical bill, the cause of action could be based on a written contract, open account, or perhaps "account stated". In some States, the different causes of action have different statutes of limitations. If suit is not filed within the applicable period, a later action will be barred. Note though, that a partial payment made sometime before the statute of limitations runs will "restart the clock".
I am not licensed in CO and this is a general discussion only. It is not intended nor should it be construed as legal advice.