From the blog at silversolds.com
While this is specifically about Stager Silver Solids pure silver interconnects, it applies to most cables:
"There should not be a need for a long, extended break-in period, as many cable makers claim, actually hoping your ears will adjust.
I’ve gotten several different opinions regarding break-in from different users. A few, whose comments I posted, found that no break in was necessary. The cable should sound fine from minute one. Improvements, if any, are generally very subtle.