I assume you are talking about laying hens?
Of course you can get different opinions on this but I’ll try to explain how I go about it.  A hen normally lays really well her first season, goes through an adult molt, and lays really well her second season.  After her second adult molt her production normally drops.  Each hen is an individual and you can get a lot of different individual results, but the flock average normally drops about 15% to 20% after the second adult molt.  After each successive adult molt you get a pretty significant drop in egg production.
Often, not always but often, pullets that start to lay in the fall will continue to lay throughout the winter and keep laying until they molt the next fall.  All this assumes you do not manipulate lights.
It took a while to set it up but I go through a rotation.  My ideal flock size is 6 to 8 hens so each year I keep 3 to 4 pullets so I get eggs throughout the winter and have replacements.  So once these new pullets start to lay in the fall I may have 9 to 12 hens and pullets laying.  When the adults molt and stop laying, I keep the previous year’s pullets and feed them through the molt.  They will come back laying really well after the molt.  But once the molt starts and I’ve gotten all the eggs I’m going to get that season, I butcher the oldest hens.  Each year I replace half my laying/breeding flock.  
Hopefully this makes sense.  Others use other methods, there is hardly ever just one way to do something.