What is the name of your state (only U.S. law)?
texas
How long does a criminal trespass order last in texas without due process?
As it is merely a warning and not a court order, I suppose it lasts until the property owner wants to withdraw the warning.
This notice - a criminal trespass warning (or "order" if issued by a court) - appears to allow for the fulfillment of the "notice" requirement of TX Penal Code section 30.05 with regards to trespassing.
Since you do not have a right to trespass on another's property, I suppose the warning is valid forever or until withdrawn. You will have right to due process when/if you are arrested or charged and go to court. At that point you can argue whether or not notice was properly given.