I am unable to answer that question with anything close to an actual number.
We started our flock with a foundling cockerel. Once he had a safe abode for himself and a number of hens, we went out to buy 3 hens for him. Although I had done a ton of reading on what to do and what not to do in order to keep chickens properly, I knew nothing about guessing the age of a hen. And, Man did the seller see me coming.

We bought 3 hens, 1 was a Mille flore something, that was a full size hen. She was beautiful and friendly and I named her Ursula. I will never know how old she was when we got her, but she was my pride and joy. She never laid an egg, ever. So she was past laying age when we got her and we have had at least one hen who laid fewer and fewer eggs until she was 7 years old.
We had Ursula for about 8 years, I guess. So she would have been at least 10 years old and probably 15 or more. She was and always has been missed. One day perhaps I will tell a few Ursula stories, she was rather bossy, even with the Rooster.