How long? I have noticed improvements in my components for about a thousand hours. I have been pursuing high end audio for fifty years… I have found varied burn in times vary among components, but are highly reproducible. Those within a line of components from one company are pretty consistent. Over the last year or so I broke in five Audio Research Reference components, three of them 160s amps. The sequence and timing from all were nearly identical… the amps exactly the same.
One of my major interests is consciousness and metaphysics / epistemology (which is what led me to an interest in consciousness and as a consequence evolutionary, developmental, and abnormal psychology. So I am very familiar with the brains ability to be preconditioned and fill in perceptions. Armed with this, I still give no credibility for break in… and a number of similar observations being psychological.