Necklace length is very much a personal choice and some lengths will just "feel right" when worn.
If you have an existing necklace that is the perfect length, that makes it easy. Just unclasp it, stretch it out to it's full length and measure with a tape measure. If you don't have a necklace to measure, you can use a piece of string, ribbon or soft tape. Drape it around your neck until you have the right length, mark the spot where the two ends met and measure it with a tape measure.
The "Choker" length (40 cm) sits close against the base of the neck. This length looks good with a wide range of clothing as it can be seen with anything you wear.
The "Princess" length (45 cm) is the most commonly worn necklace length and sits on the collarbone. It looks good with a pendant attached which creates a "v" shape
The "Matinee" length (50 cm) sits between the collarbone and the bust. This is a good length for business or casual wear. This length works particularly well with collared shirts. If you are tall, then this is a better style than the "princess" length.