Mercury is a trademark, not a copyright. As long as Ford takes steps to protect the trademark, it maintains the rights to Mercury indefinitely. An extreme example of protection would be to take legal action if some other company tries to use the Mercury name on a vehicle.
Manufacturers of car model kits in recent years have had to get permission and pay royalties for the use of long-disused names of vehicles on kits and packaging. Chrysler in particular maintains rights to numerous old names because of its own history and purchasing American Motors. One example is Nash. Chrysler does enforce those rights on the model-kit companies.