Hens can be expected to not lay on a schedule, though if you find they are laying significantly less or even not at all, something may be influencing them to do so. There are actually many factors that can cause a hen to decrease laying or stop for a while.

It takes about 14 hours of sunlight for a hen to produce 1 egg. In the wintertime, daylight hours decrease, and so do the eggs. This is a very common occurrence. Some people prefer to add artificial lighting to trick the hen into thinking it is sunlight, which would cause her to lay more eggs. Similarly, some people prefer not to add artificial lighting so that the hens can take a break from laying.

There are many things that can cause a chicken to experience stress, and therefore decrease laying. Predators, a change in environment, flock bullying, lack of privacy, and weather are just a few factors that can cause chickens to feel stressed.
If a chicken feels threatened or stressed, they will not lay as usual. Try to minimize any stress they may experience, and create a comforting and safe environment for them.

Proper nutrition is key to healthy chickens and great eggs. If a hen has a diet that is not nutritional, she is likely to lay odd or less eggs. It is advised to feed your flock a complete layer feed of at least 16% protein, and also make sure a calcium supplement (i.e. oyster shell, ground up egg shells) is also always available. Also be sure to not feed your flock too many treats, as they will overeat the treats and not eat enough of their regular food.

Something that can't be prevented, age is sure to catch up to your birds sooner or later. It is typical for a hen to begin laying eggs at 6 months of age, and start to decrease laying at about 2 years of age. However, some hens continue laying occasionally into 5 years old. This factor is dependent upon each individual hen, and when her body will run out of ova to produce eggs.

(Above: At 6 1/2 years old, Hennrietta laid occasionally.)​

Often a first sign that a hen is sick is when she stops laying or lays less eggs suddenly, soon followed by other symptoms of an illness. An illness can take a toll on a chicken's body, which would sometimes impact laying. Egg-bound hens (a common problem), as the name suggests, are not going to lay since an egg is stuck in them.
Treat any ill chicken the advised way for that particular illness. If a hen has been on antibiotics, it is advised not to eat the eggs from her for at least 2 weeks, and never to eat her meat.

The seasonal shedding can leave your chickens looking bare, and your egg cartons too. Molting puts an added stress on chickens, and hens will often stop laying, even if the molt is small. During a molt, the chicken needs all the protein it can get to help regrow feathers, so eggs are not a priority for the chicken at this time. She will typically resume laying about 2 weeks after her feathers have come in fully.