AFAIK, a dealer-owned car has “dealer” plates and isn’t formally titled. Once it’s sold, it’s titled to the owner, and (assuming the owner lacks a dealer’s license) it has to be sold as used–even if the owner puts it on a car carrier and delivers it directly to a third party.
I don’t know if there are statutory limits…but in the absense of any, I guess a dealer could hold on to car for a decade, put 100k mi in it, and sell it as “new.”
They may be sold as new in some states but that’s more a determination of state law nomenclature and sales puffing than anything else.
In some states, a demonstrator can be considered both new and used depending upon how the law is viewed and some states may have added their own tweaks.
The FTC has a rule on this; condensed and cut and pasted as below.
What used vehicles fall under the FTC Used Car Rule?
Any vehicle that meets the following criteria, whether it has been titled or not and driven for purposes outside of moving or test driving (in which case it can be removed, but must be returned after the test drive is complete):
Gross vehicle weight rating (GVWR) below 8,500 pounds
Curb weight below 6,000 pounds
Frontal area less than 46 square feet