a dog's age
An exceptionally long period of time. It's been a dog's age since I worked one of these machines, but I'll give it a shot!
dog's age
see under coon's age.
a dog's age
A long time. An American slang term dating from about 1830, this expression doesn’t make a great deal of sense, since the average dog is not especially long-lived. It appeared in print in 1836: “That blamed line gale has kept me in bilboes such a dog’s age” (Knickerbocker magazine).