There is no such requirement. They can sell it the next minute after you trade it in if they have a customer.
I would think they are allowed to keep them for as long as they wanted to. Whether they are required to keep them or, not is another issue, that I do not have the answer for.
Infinitely long
I don't know but you might want to check out the following site: http://www.consumeraffairs.com/automotive/dealers_nv.html
How long should a medical record be retained
not reqauired
There is no length of time required before you can trade in a car. Most people wait around one year before trading it in.
A long history of inbreeding and war provocation to keep the empire growing and to keep the opium trade viable.
for 5 years
You might want to ask the dealership that question.
Investigators are typically required to keep consent documents, correspondence, and research records for a minimum of three to seven years after the completion of the study, depending on institutional and regulatory requirements. It is recommended to check with the specific guidelines of the institution and the governing regulatory bodies to ensure compliance.
You trade it in for a new one, As long as you have a permit. This only applies if you live in a country where a permit is required.
"http://wiki.answers.com/Q/How_long_it_is_required_for_a_dental_provider_to_keep_past_insurance_explanation_of_benefits"