Chickens will lay as long as they live although at a reduced rate. The Univ of Florida Ag Extension service has a graf that shows a hen decreases her egg production about 10%/yr after her first full year of laying...i.e. a five year old hen will lay about half the eggs she laid in her first year. A normal heavy breed of chicken will lay about 240 eggs her first year so a five year old will lay around 120 or about 2-3/week. Many things affect egg laying such as diet, molting, stress, access to water and light levels. Typically an older hen will stop laying during the winter with low light levels and resume in the spring whereas a one to two year old hen will lay through the winter.
Guiness World Records lists the oldest hen at 15 years old but the average full size hen is normally good for 6-8 years in my experience although there are members on this forum that can claim longer lived birds. Unfortunately, there are a lot of medical problems that can shorten our dear birds lives...just look at the disease/sickness forum. Hopefully your girls will be long lived.