I ran an I7 920 at 4Ghz for roughly 2 years at around 1.35V with water cooling (max was 55C on full load on all 4 cores). After the 2 year mark the chip became

I ran an I7 920 at 4Ghz for roughly 2 years at around 1.35V with water cooling (max was 55C on full load on all 4 cores). After the 2 year mark the chip became unstable and over the period of the next 6 months the clock speed it would reliably maintain was 3.2Ghz. On that progression it would be down below its default rated speed of 2.6Ghz in presumably another 6 months or perhaps outright failed. The current estimate is that Intel targets about 15 years for a CPUs life at the clock speeds they ship, overclocking can vastly decrease that.