Everyone is different - different sizes, shapes, preferences, styles... There is no one answer as to how long a chain should be. How long do you want it? How long do you like it? What effect are you looking for? What feels right for you?
Having said that, some lengths are much more common than others. As some semblance of guideline:
● the most common necklace length is 18 inch (princess). This is probably the go-to length for day-to-day wear.
● the 'most flattering' necklace length is said to be 16 inches (choker)