How long does it take to run the function constructed by for loop in dsp?

I think it's just a simple CPU idling. If you want to calculate how much time it takes, you should be able to estimate the approximate time.

2000*count* single instruction execution cycle time (regardless of the delay gap of executing jump instruction, the reset of inner-loop counter and the accumulation of outer-loop counter).

The execution cycle time of a single instruction is the reciprocal of the main frequency of the DSP processor you use.