Do cables settle in?

IME, all systems take a while to settle and sound natural after moving pieces around, hence at audio shows, the first day is never the best sounding, the second or last days usually far better..

In my experience, they do settle in, and it applies to interconnects as well as speaker cables.

Without a doubt .

They settle in about 20-50 hours then they sound better big time!!

IMHO...yes. George Cardas is a big proponent of this and I've actually heard him demonstrate this several times with his cables, (I was a Cardas dealer for many years).

http://www.cardas.com/insights_break_in.php

I've got better idea how to waste your audio leisure:

For critical listening wrap your cables with surgical ice wraps that you'd store in the freezer. It can hold the freezing temperature for long hours and when it becomes worm(er) you can put worm ones back in the freezer and pull out fresh cold ones and re-wrap your cables.
Please note that WAF factor may intervene your endeavor especially if you overpopulate one with substantial number of these freeze sacks :)!

Some times yes and some times no.
I've had it work both ways depending upon the cable.