How detrimental is overclocking to the PC? How much voltage is too much?

There is no particular 'range' it all depends on the individual system and not using voltages that are far and above what is needed - YOU need to do the testing, ensure stability at lowest voltages possible

From what I've experienced in over 25 years of PC building, if done properly None. The systems end up get getting to slow for productive work. I have yet to have a system I've built die prematurely from OCing

If they made a cpu im and should last like 10 years it may only last 7. Use that for an example to be honest, most people will update the cpu before you even notice degradation. Degradation on a cpu is really over exaggerated on forums. If you're on amd and running 1.55v maybe you get 5
instead of 10, if 1.45, maybe 7 years, numbers aren't too be taken super accurately cause there's a lot of factors but its just an example. To summerize, as long as you don't go crazy, you'll have a new cpu by then most likely

Start by doing the research for the chip you intend to OC. I have the 4690K I'd look to "business" OCs first http://www.digitalstorm.com/unlocked/intel-devils-canyon-i5-4690k-review-and-overclocking-benchmarks-idnum301/ and http://www.anandtech.com/show/8227/devils-canyon-review-intel-core-i7-4790k-and-i5-4690k/2 are a couple of those. After I exhausted the business overclocks I would turn to other users of the same chip like http://www.tomshardware.com/answers/id-3249593/overclocking-4690k.html . I see answers about over-volting such as http://www.tomshardware.com/answers/id-2739638/4690k-overclock-max-safe-voltage-temperature.html . Never should someone accept one person's or business's advice when it comes to PC decisions, especially OCing. That being said one should ALWAYS follow the manufacturer's recommendations. Most K version CPUs are sold without a stock heatsink. Intel understand we that purchase a K version chip will most likely overclock it. Even though that is true the Intel warranty is void should a chip fail during an OC no matter how mild.

As I was studying and seeing how others did it I would perform the OC myself. I would know that no two chips, even my neighbor's 4690K, will overclock exactly the same so I wouldn't assume his entries will be identical to mine. Assuming where OCing is concerned is dangerous. After I had OCd my 4690K I would pay close attention to my temps and other information my health monitors provide.

My regular TV passed away(may it rest in peace) over a week ago so I'm using my back-up CRT.

I would then run Intel Burn Test and Prime 95 for about 10-15 minutes to ensure stability. I would monitor the temps in real time to make sure it wasn't getting too hot. Some argue against those stress testers, saying they are unrealistic and you should just run your most intensive program for real world results.

There is no way that my results mean a thing to someone running a 6700K i7 or even another 4690K user. As such there isn't a blanket answer to your question.

In general as long as the OC is done responsibly I don't see it being detrimental. Going out on a limb it may be helpful. Many who OC use aftermarket coolers meaning their temps are normally cooler than regular users. They are usually more in tune with their PC and can take care of it better than someone who simply uses their PC. More informed...

Your cpu can be oced and running 20c all the time, as long as there is more voltage it still has a negative impact. Heat isn't everything. The increased voltage has just as much of an impact. But like other people said, as long as its responsible you'll be fine. Don't stress yourself with worrying about every little thing when ocing. Have fun and becareful that's all.