The "burn in" is not a fixed amount of time. Different vendors do different burn in's. e.g. the individual components may be burned in for a certain time (and it's basically what the vendor wants to use - whether it's an hour, 12 hours, 24 hours, or merely a test to make sure the board is working). When a vendor gets the parts, some have QC burn in's and others do not, again it depends on the PC vendor and sometimes the particular component. Once the PC is assembled, it is tested and may be burned in, again depending on the particular PC vendor or depending on the model (e.g. severs and commercial PC's may get more burn in than a consumer PC).

According to Dell tracking, when I ordered an 8200, the parts were "picked", the PC was assembled and it was shipped on the same day. If the Dell tracking was correct, I'd say it didn't get much if any "burn in".

However, where I worked (I'm retired) at a large Federal Government agency, we had a contract with an independent PC company for about 4000 PC systems. I went to the vendor's plant, along with some others, and on the "50 cent tour" we were shown their burn in processes, which included a 12 hour burn in and testing of incoming components that they didn't make themselves (they had a plant in Taiwan and made their own motherboards which were burned in before shipping them to the US). Once the PC was partially assembled it went through another 24 hour burn in cycle and then when the PC was completely assembled it went through a final 24 hour burn in and QC cycle where 3 different burn in programs were run on the PC.