How long for an HDMI cable to break in.

2 seconds.

@1971gto455ho  Have you got a Craig floor mounted 8-Track in that puppy?

Freezers don't get that cold. So, you need to travel on reindeer back to the frozen north of Canada, Russia or even some place near Barrow Alaska for cable cryo treatment. Bury it in the permafrost and keep it buried for 2 years.

Play this ritualistic audiophile track during your cryo treatment travels to the yukon.

https://youtu.be/tzdasXF9H0U?si=E0Gq0ik8TX27WNiS

When the 2 year mark is up, retrieve it on reindeer back.

Is it ready to go now? Not only no, but, hell no.

Now, your 4000 hours of hdmi break-in commences (as mentioned in other comments). 

@deep_333 my freezer is plenty cold to get the vodka to its optimal operating temperature 

traveling by reindeer to cool it is not an option - patience has never been my virtue 

OP, keep the more expensive cable in your system for a few weeks, with signal running the whole time (just turn vol on pre) and after a few weeks swap back.  If it's still subtle or you are liking the old one, immediately return the $600 cable.  I would think there would be a difference right off the rip if it was higher quality but the connected components/system as a whole could make a big difference as to audibility of these types of cable changes.  I never change an audio component unless the change is not subtle, and repeatable.  If you have a friend to blind test you, even better (not that blind ABX is without its flaws but you should be able to denote the better cable most of the time)