The life span of a (any) derailleur will be determined in large part by the conditions under which it has been ridden and by how well it has been maintained. I've seen some high milage well worn rental mountain bikes with Shimano Altus equipment that shifted just fine.
In general if you keep derailleurs lubed and reasonably clean and protect them from road rash they should last for a very long time.
Last edited by Delmarva; 06-17-16 at 09:59 AM.