EXCELSIOR CLASS: 80 YEARS
The first Excelsior class starship, the Excelsior itself, was seen in Star Trek III, Star Trek IV, and Star Trek VI. Its service life lasted longer than the entire service life of the Enterprise NCC 1707-A (at least its entire service life with the designation of Enterprise NCC 1707-A since it might originally have had a different name and number).
A number of Excelsior class starships appeared in Star Trek: The Next Generation, DS9, and even in "Endgame", the last episode of Voyager. According to the official chronology that is from 2283 to 2378, or 95 years.
Of course individual Excelsior class starships may have served for far less than 95 years. The Excelsior class starship USS Repulse was seen in 2365 ("The Child") and mentioned on more or less readable screens as late as 2374 ("In the Pale Moonlight"). Its register number of NCC 2544 is only 544 digits higher than the NCC 2000 of the original Excelsior, but many other starships operating in the next Generation era had register numbers tens of thousands of digits higher, thus implying that the Repulse should be almost as old as the Excelsior.
So many persons may interpret the data as indicating that Excelsior class starships sometimes serve for 70 or 80 years.