i'm tall (5'10) so for me 16 inch hits me in my collarbones kind of hiding the pendant so I wear an 18inch normally or 20 to 22 inch for a bit more length so it hits above my chest. I think necklace length depends on your height as well as neck too.
I'm 5'4" and prefer to wear 18" chains but I also own 16" ones too. The 18" feels more comfortable around my neck, whereas the 16" feels short or feels kind of tight on me. I switch up the chains depending on the necklines of my clothes for each day.
I'm 5 feet tall and on me 16 inch necklaces lay more like I'd imagine 18 inch ones would on more average height women. I have 18 inch ones and they hit just above my chest. I wear both but prefer 16 most days.
It's all really subjective and for me it also has to do with shape, size etc. and everyone is different!
For example, to get the look that say canyongirl (I really hope you don't mind using you as an example!) has with her lovely 18" chain and key pendant, I just know straight off I would have to use a 20" chain to have the same effect.
Imho, the Tiffany chains of 16" are too short to be sold as a 'regular' length - it's very high on the neckline but that's what some people might want, but personally it's not for me. I recently bought a Tiffany pendant and went for a 20" chain which I'm very happy with.
A good dressmaking tape measure is your friend in these situations as are any necklaces you might own (and measure to get some sort of comparison).