Something like this:
time_start = micros();
fix_fftr( fx, FFT_SIZE );
time_fft = micros() - time_start;
You checking time - micros() before and tight after some procedure, than printing out via serial monitor your results.
Arduino scetch as an example , linked at this page:
http://coolarduino.wordpress.com/2011/09/03/arduino-musical-note-recognition-pushing-the-limits/