since Hector was a pup. For a very long time.

Since Hector fought in the Trojan War over three millennia ago, in some ways this is certainly longer than a dog's age. Or is it? No lesser lexicographer than Eric Partridge tells us that when this became an American catch phrase in the 1920's Hector was a common name for a dog, especially a mastiff. So while the Trojan prince may have been one's dog's namesake, the time since the Hector in question had been a pup might be a matter of just a few years.

Michael Quinion of World Wide Words takes it a bit further: “Anyone versed in Greek mythology…would have remembered that, according to Euripides, in later life [Hector's mother] Hecuba was turned into a dog for blinding Polymestor, the murderer of her son Polydorus, so you might consider Hector to have been a literal pup, perhaps even the original son of a bitch.”

since God's dog was a puppy. Indicating a condition that has been in existence from the very beginning. This is, of course, much, much longer than a dog's age.

I came across this phrase in Lee Child's first Jack Reacher novel, Killing Floor. An elderly black man in a state prison in Georgia says, “I've been in this joint since God's dog was a puppy, yes sir. Since Adam was a young boy.” Google reveals that his is not the only usage. While the character in Child's book is from the U.S., the author is British and does not always get the vernacular quite right, so I have reached no conclusions as to regional or cultural origins.

About the illustrations: Figure 1 is a detail from an Attic amphora (a double handled vase) by Euthymides ca. 510 BCE. The decoration shows the arming of Hector by his parents Priam and Hecuba, an event which suggests that Hector was still young, or a “pup,” at the time. This image is excerpted from an online educational site, Classics 1020, and the copyright for it is most likely owned by the publisher. I believe that the use of scaled-down, low-resolution images to provide critical commentary on the image qualifies as fair use under United States copyright law.

Figure 2: Few art historians have given much attention to this dog at God's side in Michelangelo’s depiction of Him on the ceiling of the Sistine Chapel. We note that as He is passing on life to Adam, the dog appears to already be well past puppy stage. Collaged by the author.