How long for an HDMI cable to break in.

Drink 48 ounces of blueberry vodka, then test again and report.

Absolutely great thread! Since the first page, it’s been nonstop chuckling with the morning coffee.

Five years, 2 minutes, and 4 days. Roughly

I am still confused what makes the difference in (HDMI) cables used for i2S connections between streamer/transport and dac. Is it the materials used, the length, the shielding? Is a dedicated i2S cable I general better than HDMI? 

/stievus Short Answer:  NO It's an HDMI cable, simply adapted for I2S use.

Longer answer.

I²S most commonly uses 3- 4 pins with an optional 5th depending on the design.  

SD (Serial Data) carries the audio data stream.  
SCK / BCLK (Serial Clock / Bit Clock) synchronizes the data bits.  
WS / LRCLK (Word Select / Left-Right Clock) indicates whether the current data word belongs to the left or right audio channel.  
MCLK (Master Clock) optional; provides a higher‑frequency reference clock for DACs/ADCs that require it.

The notion that an HDMI used in an I2S format or in its original use state requires a burn in or break in lacks an reference to electrical properties science. It needs no longer than the time it takes for you to plug in both ends and send a signal across it.

Digital transmission: I²S carries raw audio data as 1s and 0s. Either the bits arrive intact, or they don’t. There’s no gradual “improvement.”  No "dielectric forming" Unlike high‑voltage analog cables, the low‑voltage differential signals in I²S never stress the insulation in a way that changes its properties.