I have necklaces in almost all lengths for different outfits or for layering. So I think it depends a lot on the outfit - sometimes an 18" is perfect, and sometimes it just flips in and out of the neckline of a top or dress, which doesn't work at all. I like long 30" necklaces to layer, too, but they can get lost in a cardigan or scarf. So I think having different lengths is a must.