Chickens can molt at any time, usually due to some type of stress. Most of these are mini-molts where just a few feathers are lost. They usually either quit laying or greatly cut back on laying when they go through these mini-molts and use the nutrition that usually goes top eggs to grow new feathers instead.
Normally when the days get shorter in the fall the chickens go through their annual molt. This is where they lose all their feathers, usually starting around the head and moving from there. Feathers just wear out and need to be replaced. Different chickens molt at different speeds. Some finish in less than 2 months, some take up to 5 months. This speed is not about how fast the feathers grow back, it’s how fast they fall out. When they go through one of these complete molts they stop laying eggs until they grow the feathers back.
Often pullets will skip the full molt their first year and continue laying until the following fall and they start their first adult full molt, but they will probably go through a couple of mini-molts that following spring and summer. Their bodies just get won out from continuous egg laying and need some short rests.
Since what triggers the full molt is the days getting shorter, you can use artificial lights to keep them laying and get them to skip the molt, but after a certain amount of continual laying, say around 16 to 18 months, their bodies get so worn out that the production really suffers. Instead of laying 6 eggs in 7 days, she may only lay 3 or 4. The quality of the eggs suffers too. Shells might be thin, whites might be watery. That’s why the commercial egg laying operations force a molt, usually when production drops to about 60% of peak production. It just costs too much money for feed to get the poorer quality of fewer eggs.
Some hens will start to lay as soon as their molt is finished, some wait for spring and warmer weather. Most production breeds tend to start laying again fairly soon after the molt but the more exotic decorative hens are more likely to wait until spring. But this varies by the individual hen. There is no firm rule, just a tendency.
