It's both. Some pieces hardly need break in time. Meaning I can't tell it sounds better after 100 or more hours. But, I have some interconnects that sounded horrible for about 200 hours and then BOOM! Everything changed in one day.
Are You A Disciplined Audiophile?
The issue of whether break-in is real aside for now, when you make a change in your system, such as a new component or cable, do you have the discipline to wait before making any other changes? I usually mark my calendar for a month and perhaps 2 months and try to change nothing else for that period of time so I can better assess exactly what the new thing is doing. But sometimes it’s difficult to wait. IMO, break in is a real thing, both in the component and the listener, but even if you don’t believe components and cables change after a few days, can you wait at least a month to listen to enough music to adjust your ears to what the new thing brings to the system on its own?