Depends on the breed, the conditions she lives in, her age, her general health, etc. There is no set time. I do think, in general, she will begin to slow down toward the beginning of the second year of life, but will continue laying for years to come, if she is well. I have some who are four years old and though they have eased off some, they still lay several eggs a week when not molting. Someone here had a hen up in her teens (17, I think, when she died) who still laid an egg every so often.
most of my chickens lay consistently until around 4 or 5.... Then I often see a steep drop off in egg production. Even the old gals lay sometimes. A very old Speckled Sussex that I am not sure of the age of (I estimate her to be between 7 and 10 years old) because someone dumped her in my yard, layed very well until this year. Almost daily! What a chicken! I would be curious if any other sussex owners have birds that laid a long time like that.