Tar can last from months to many years depending on the type, surface, exposure, thickness, and maintenance. In archaeological conditions, some tar-like residues can survive for thousands of years.
Tar longevity depends on context. A thin coat on a sunny fence may weather, dull, and need renewal. A heavy sheltered coating may remain for years. A tar stain absorbed into timber can be almost permanent visually, even if the useful protective surface has worn away.
Traditional wood tar was often part of a maintenance cycle, not a one-time permanent cure. Surfaces could be retarred when they dried out, faded, cracked, or lost water resistance. That renewability is one reason Stockholm tar and pine tar remain interesting for heritage work.
Coal-tar and bituminous products can also last a long time, but durability should not be confused with suitability. Some old tar-like coatings persist because they are stubborn and chemically heavy, not because they are desirable or safe to keep exposed.
The broad answer is that tar can be very persistent, but its protective life is shorter than its stain. A surface may still look tarred long after the coating needs maintenance.