Expected usable life of any 24VDC power supply if continuously being in use in DCS & PLC System

What is expected usable life of any 24VDC power supply if continuously being in use in DCS & PLC System ?
Since product datasheets of renowned make power supplies define MTBF more than 3,50,000 to 5,00,000 hrs, but that is not the practical case. Any documented evidence will be helpful.
 
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