Another potential problem is that chickens have a natural laying cycle. The natural cycle is that they lay well in the spring, summer, and maybe early fall, but then the stop laying, molt, and recharge their system when the days get shorter. The in the spring when days get longer they start the laying cycle again. A lot of people manipulate that natural cycle by managing the lights, so this is not just seasonal.
You still have to take care of chickens and feed them whether they are laying or not. Some people don’t want to spend their time and money taking care on hens that are not laying, so they sell them. Let the new owner pay for the food and spend the time and effort managing them while they start on a new flock. This can take months and it can be any time of the year, depending on how the lights have been managed.
As others have pointed out, sometimes there is practically no drop-off or they get over it pretty quickly. Sometimes it takes a while, days, weeks, or even months. It’s hard to come up with an average that covers all cases.