Standard necklace lengths:

First, you need to understand the exact look you are going for with your necklace using the chart of standard necklace lengths. There are roughly 4 standard looks are that are based on variants of necklace lengths, the most common are choker and princess lengths.
Length - Inches Length - CM Style Worn best with
17'' 43cm Choker Almost everything
18'' 46cm Princess Almost everything
20'' 51cm Matinee Great for both business and casual wear
22'' 56cm Opera High necklines and evening wear