When is a chick a full grown chicken??

Depends on the breed. Some egg production breeds are done with their growing by 6 months or so. My LF brahmas are still filling out and gaining weight. They're 13 months old.

 

Fully grown... hmmmm. It kinds depends on how you define it.

A cockerel is ready for slaughter at 12 weeks, so that could be one option.
6 months has always been considered the norm, for pullets. But there is no single answer for such a question, as much depends on breed and conditions. The norm for one may not be the same for another.

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Chicks are only chicks for about 4 weeks. After that, they are pullets or cockerels.

Traditionally, the first molt marks the turning point from pullet to hen for females.

Males are generally considered to go from cockerel to full fledged cocks after 12 months.