It varies wildly from component to component. Tubes Typically need between 60 and 200 hours to sound right. With tubes, there is an outer coating of material that needs to burn off and negatively impacts performance. For most tubes, 60-100 hours is plenty. It is only 300Bs and other tubes that can last decades that usually have really long burn in times.
Cables need to charge the dialectic. Parts need to get hot and cool off repeatedly. The spider on a speaker needs to flex.
Some of these changes can be significant. Anyone who tells you it is just you getting accustomed to the new sound has not had a lot of new gear. For those of us who are perpetually opening new boxes, it varies wildly from manufacturer to manufacturer. When you get a new device, your retailer or the manufacturer should have a POV.