I would assume the max core temperature is the TjMax used to calculate your core temperature. That is the point where the temperature delta value read from the core sensors reaches 0 and the CPU starts throttling. So a safe max core temperature would be around TjMax-20 °C or TjMax-30°C.
If you use the Open Hardware Monitor to read your core temperatures you can find the TjMax parameter used by double clicking on the core temperature sensors (or right click -> Parameters...).
You need to bee more specific... Max core temperature refers to Tjunction Max, which is the temperature your processor will start to throttle down in order to avoid damage, somewhere around 100c. Your more than likely talking about max "safe" core temperature, which is in fact around 70c.
For 24/7 operation, I would recommend keeping under 60c-65c. Benchmark/stress test to 75 is ok. Other than that, your good to go.
Obviously Intel disagrees otherwise they wouldn't have made it 105C. But you have to remember that this is Tjunction Target and not Tjunction Max that you're actually reading from Core Temp. Tjunction Max ought to be the same or higher but it will vary slightly for each core. Tj Max variation plus sensor accuracy issues mean that unless you're running really hot there's only so much you can trust these things. After all, they were never meant to report accurate temps.