What is the rate Chicken egg production decline by age?

My chickens are in an open field and lay eggs in several multiple nest box structures [12 nesting cubicles per structure]. I can not determine which hen is laying or not. My hens lay eggs in nest box where others have lain/laid/dropped their egg. One box will have 5 eggs and another none. As my hens age, they will probably produce less. I understand that hens normally on average lay an egg every day and a half on average when they are at peak production. [I think the record is seven eggs in a day!] My question is what production ought I expect as my hens age? The answer will help me calculate a hen replacement scheme—although I will keep all my hens until they drop naturally. I want to maintain egg production at about 50 to 60 eggs per day. Thanks!