How long should my table runners be if I'm using a 72" tablecloth?

If I have rectangular tables & I'm using 72" tablecloths... How long should I get the runners to be? Should they fit exactly to the end of the tablecloth? Do I go longer? But how long is too long? I don't want it to look messy or tacky. A lot of the websites I've been going on have runners at about 108" long. It's been hard to find them at 72".