Depends, layers at the factory farms are pushed hard and Agboy's reply is pretty accurate.  They will push them hard for almost a year, then force molting for 6 weeks and push them again for one last hurrah so to speak.

Free range chickens like my dad raised when I was a kid tended to molt about twice a year.  They were not pressed hard to lay and seemed to all molt about the same time so egg production during that period slacked way off.  By 3 years of age you noticed an increase in death rate and by 4 years of age less than half of that group would be left.  Time to butcher for chicken soup before they gave up the ghost.  Every year dad kept a hundred pullets for replacements.  Had 350 to 400 chickens during the 60s and 70s.