In my experience, audio rack burn-in time varies depending on many variables.
Is your rack made of wood, glass, or metal..........or a combination of the three?
Wood component racks create the warmest sound, but they have a longer burn-in time. It's also true that hardwoods burn-in faster than softer varieties....IE.....oak verses pine.
Metallic audio racks burn-in faster, but they tend to be more bright sounding. Their color will also determine just how bright. IE... silver metallic racks sound brighter than black audio racks. It's also true that if your components are placed on glass shelves, the audio becomes more transparent.
Never underestimate the influence of component rack materials & burn-in time!! Burn-in aside, you should always attempt to give your audio-racks an "in home" audition to make sure you get the one that sounds best in your particular room.