It depends on the breed. A rule of thumb is around 20 weeks, or up to 26 weeks or so, but I've heard of a hen not laying til she was a year old, and of ones who started at 14 weeks. They may never lay a fart egg; I've only seen one. At first they lay "pullet" eggs, which are smaller than what they will lay for most of their life. For large fowl, pullet eggs are around "small" eggs in the grocery. The eggs gradually get larger; usualy after a month or two, they will be laying their regular size eggs. Then after they are 2 or 3 years old and laying fewer eggs, they will probably lay really jumbo eggs.