depending on your ear, or what you would consider broken in, I have found that there are actually different levels of "broken in". When cables are brand new and sound real grainy and flat, and then they make that "transformation", I think THAT may be a one time thing, and that is maybe, because I have noticed that evan a cable that has just had a major breaking in period, if that cable is then switched to another component, it still will take some time to settle in to the new connection, although the effect is less dramatic. Cables that have been idle, seem to break in more dramatically than cables that are merely switched, but I think the time period may be about the same. Mrowlands is definitely right about cable dependancy, and I would add that some cables "age" better than others. I have noticed that some old cables don't sound as good as a brand new one that is identical, so the effect of contanimation (tarnishing) or evan years of lots of currant running through it could have effects of varying degrees. I can't be sure of aging due to currant, but I am sure of the rest of the above in my experience, and here is my cable burn-in guidelines I use;
Interconnects, copper; new, 48 to 4 days continuous, idle or used, 8 to 36 hours, reconnected, 1 to 6 hours.
Silver IC's new; 24 to maybe 3 days, maybe less than copper on idle or reconnection,
Speaker cables, idle; 16 hrs, no change after 1 week,
To see if I like a cable in a set-up, I give 24 hrs for a new interconnect, 4 to 8 for idle or used, and only time to listen for a reconnect, that is, if I just burned it in, I don't need time to see if it works. Why less to see if I like it? Because I have never heard a cable change sufficiently to change in character beyond that, and while I don't care much if it's my own cable except for curiousity, if it belongs to a dealer, it's gotta get back.
