How much electrical wire loop be left on wires before entering the outlet and switch box?

From where the wires enter the junction box there should be 6 inches of wire left for the electrician to complete the splicing of the circuits.

NM cables must be supported within 12 inches of a box. There is no loop of extra wire allowed such as you might find with data or low voltage cables.

Most conduits must be supported within 36 inches of a box.