Uh, I think you don't want your CPU fan turning off, especially if it's the stock cooler. It should only turn off if you power down your computer.
The fan doesn't usually turn off. It only slows down. This is to save power, since at idle, CPU doesn't produce as much heat. The fans are rated to run at around 40,000-50,000 hours, so that's around 4 years of nonstop usage.