Can a chicken egg be fully formed inside another?

It takes 24 hours.

The yolk, which is called an oocyte is produced by the hens ovary in a process we call ovulation. As the yolk is released into the oviduct it is fertilized by sperm deposited by the rooster up to 10 days prior to the ovulation.

Fertilized or not, that yolk continues down the oviduct and is covered by the vitelline membrane, structural fibers and albumen in the magnus area of the oviduct.

As this forming egg continues it's travel down the oviduct it rotates continually within the spiraling tube (oviduct) and this movement twists the structural fibers forming rope like strands called chalazae. two of these chalazae anchor the yolk in the very center of the albumen.

As this egg gets closer to the end of the oviduct, the egg shell begins forming. Calcite is deposited around the albumen and yolk and in some breeds, bile ducts deposit color, which is why the egg can be white, brown or even green or blue.

Once this is all done, the egg is laid by the hen through the cloaca also known as the vent.

All this takes about 24 hours.