The lifetime of a cpu is hardly reconized as a problem as for most of us will upgrade every year... In a period of over 5 years it shouldn't even be dead. The only thing that kills a processor is running a higher voltage on it, so you want to keep it as low as possible althought enough as to not get unstable. Prime95 is a great program to test computer stability. Overclock it to your desires and run that program for hours on end =).
There's no such thing as a safe overclock speed, some chips could quite possibly have a problem with any overclock at all. It's all by chance really. Now it's rare to find hardware that fails within a 10% increase in speed, yet it's been known to happen. As any person that overclocks can tell you that you have to find your safe/stable overclock yourself. Push whatever your overclocking till it becomes noticably unstable, and then back down until you find a good overclock speed.