Changing the processor's speed has absolutely no effect on the life span of the processor. All processors come off the same wafers regardless of speed, and it's the quality of the particular die during testing that determines its speed. Also many that are marked as one speed are actually good enough to go higher, but the manufacturer marks them lower in order to satisfy demand for slower, lower cost processors.
Increasing the VOLTAGE of the core however does decrease the life span slightly. As long as it is running stable, it should be fine for a long time. If you have occasional glitches, then that is a sign that the voltage may be too much, or that it's simply not a good enough core to run at that high a clock speed.
Normal processor life span is probably about 10 years. If you increase the voltage to just barely where it's needed to remain stable, but it doesn't fry the processor, then you'll probably be good for a year at least.