The single turbo clock speed won't stay long and will be determined by many factors (like how many active cores and temperature). And, it's usually happened when single or two cores need boost. In Windows, you have to set "High performance" at power options to get the speed.
There is all core max speed. With that speed, you can run with the speed all day long with proper cooling. Mine is 2658 v3 QS (2.2Ghz base, 2.9Ghz turbo). It is running at 2.5Ghz on all cores when I performed CPU test. Chips like high end E5-2699 v3 (2.3Ghz base, 3.6Ghz turbo), I believe that it can run up to 3.0Ghz all cores even though the base clock is at 2.3Ghz.
Mine are running at around 30C or less at normal operation and will goes up to < 50C when 100% load. Room temp. is about 23-24C. And, I am using Supermicro SNK-P0050AP4 active heatsink.