As to a solid state amp if you have measurable or auditory changes in the performance of the amp after it has been on for 2 seconds, then you paid too much for it, whatever you paid. A design that is that unstable with temperature is not worth the paper the schematic is printed on. As to tube amps, perhaps a little longer due to the heaters. What does happen is the listener gets used to the sound, the sound doesn't change. Same thing for speaker break in past a short period of initial use - less than an hour. Cable break in is ridiculous - but a good rule of thumb is just past the time for the warranty to expire.
