I wrote up a scientific explanation of how overclocking affects CPU life here http://www.overclock.net/intel-gener...l#post14340326
. Feel free to read through it to get the basics.
As for your specific case, as others have mentioned it's really what you want, and what you feel safe with. Acceptable really depends on how long you expect your CPU to last.
Remember, at stock voltages, Intel rates their CPU's to run at ~68C for 10 years, which is a LOT longer than most people use their CPU's for.
The more you go beyond stock V, and the higher the Temp gets, the shorter the lifespan, but like I said in the linked post, you'll have no issue lasting 5 years even with large amount of v, assuming you keep the temperature at or below Intel's Tcontrol point of ~68C.
If you're an enthusiast who buys a new CPU every year and has no issue pushing it to as hard and high as they can to get the max OC. These guys are fine with a high temp if it gets them a high OC.
On the other hand, you have enthusiasts who consider even temps in of 40C under load to be unacceptably high.
Then you have OEM's who don't care about temps so long as it lasts the warranty period. There was a fuss a while back about Apple responding that 90C temps under normal usage was fine for their laptops (even I would say that's pushing it).
This different definitions of "acceptable" is why you run into so many different conflicting answers.
Tl: DR - Max temp really depends on how long you want your CPU to last and what your personal definition of acceptable entails.