impact of Intel Turbo Boost on frame rates

Hi guys, yesterday when I was benchmarking my game loop, I noticed that the cpu time spent on sleeping increases if I have a CPU intensive program running on the back ground. I guess the urge for processing power from the other program triggered the cpu to increased its clock frequency. I get similar result from both Intel i5 and i7 based machines.

My question is how can you accurately measure the time cpu spend on idle in a game loop? ??? It’s kinda important if you want to know how much more stuff you can throw at the cpu while maintain a constant frame rate.