Apart from electron migration issues and failures by excess (voltage/temperature), they're pretty long lasting
Much easier to have a failure because of something else: capacitors failing, oxidation or mechanical failure (for example, because of thermal expansion/contraction)
I've seen people complaining about a dead CPU but I can't find it right now
I design integrated circuits and one of the constraints in selecting the width of wires is to make sure that the maximum current density is below the electromigration threshold.
Overall though, given with how many computers I've worked with, CPU failures still seem rarer than Memory, Disk, Mobo, or Graphics failures. Of course it ends up being the CPU in _my_ computer that fails -.-
How long did it work for before it died?
There is still some variance in silicon, so yours may have had a defect that manifested itself after some time, I'm not sure they evaluate returned defective chips to see what happened (and if this is public info)
Also, the packaging is extremely complex and prone to the same kind of defects as other PCBs in the system.