CPU usage at 100% for several hours
If you have adequate cooling, then no. In general, your computer should be able to sustain 100% load for long time. (I've run my laptop for days under 100% load for CPUs and GPU and didn't have any problems).
Also, if your computer is relatively new (say less than 5 years old) it will shut down automatically in case it overheats.
My recommendation is to get some sort of hardware monitoring program like HWMonitor for example and run it while running your task. If the temperatures rise to dangerous levels (I can't tell you what is dangerous for you, since you provided almost no information), you'll be able to see and stop the demanding program.
Also the temperatures will rise quickly, so if they aren't dangerous after 20 or so minutes, then they probably won't reach dangerous levels.
Another thing to watch out for are low voltages. HWMonitor will provide voltage readings. They should be within ±5% of stated voltage for positive voltages and within ±10% for negative voltages. If you do overload your PSU, it should also automatically shutdown, but on cheaper units restarts and Bluse Screens of Death are possible too.
Short Answer: NO
Long Answer: Depends on several factors such as
- What do you mean by "several hours" (24, 48, 2000 hours?).
- The type of processor you are using.
- Is your machine's CPU properly ventilated/cooled?
Just make sure the computer is properly ventilated... if such computer is "newish" (newer than say, 2006), the worst it will happen is that the motherboard will automatically turn off the computer if the CPU gets dangerously warm.
A short answer will be: Yes, running at 100% will definitely damage your machine, but you will not live to see it - because it can take several years...
A CPU usage of 100% will not kill your processor instantly - if it has proper cooling. Before there were overheat preventions built into the CPU, there were several videos on the net, that showed CPU buring a few seconds after booting the machine. But these days are gone, your Core 2 will shut down, if the temperature reaches a certain level.
Other than that - the higher the temperature the shorter the life span of the product. So running your CPU on a 100% level will lead to a higher temperature in the CPU. Even the best cooling devices can not prevent the inner core from a rising temperature.
But since the manufacturers measured several parameters from your individual CPU, they were able to estimate, what temperature your individual CPU can withstand. Depending on this, your CPU was labeled and packaged.
All in all we are maybe talking about a few weeks to months difference in an estimated minimum life of a decade or more...
So I would not worry to much about this and try to get the most out of you CPU :-)