Phono cables are a real problem. Yes, break in is very real. You want well over 100 hours before drawing any conclusions on sound. 200 hours is safer. The signal strength on a phono cable is minuscule and the effect on a system can be substantial, so I recommend hook it up to your TV. Get lots of hours on it.
I have broken in dozens of interconnects and cables over the last fifty years. Heard the change on each.