HELP: what temperature will the fan spin? Overheating~

The temperature needle moved to the hot side from the mid-point while i was idle and parking my TT. I saw it moved 2 blocks toward the red zone from the mid point and slowly going back to mid-point in approx. >1min.
I searched the forum and knew about the On-Board Diagnostic codes 51 Engine Coolant Temperature.
Tested at my office's parking lot, idle until the needle at the mid-point (but the needle didn't go over this time) the temp is @ 90c (outside temp is 75ºF), I rev the engine for coupe of time, still sat in the middle. Went out and check, both fan didn't turn on. I was wondering when will the fan turn on automatically(fan is on when AC is on).

FYI:
Timing belt kits(from ECS) replaced @95k miles
I believe the sensor isn't the green one.
Car current @97k.

thanks!