I do one tempo run a week but spend 95% of the run outside the top end of my tempo heart rate training zone (averaging 185bpm for a 30-40min session). my resting heart rate is 55bpm. when i run within my recommended training zone it feels ludicrously easy, and hence is not efficent or effective training. I think its best to judge how you feel during a run, and use your resting heart rate, average heart rate and heart rate recovery as guides, relating to how your training is progressing.

i do my tempo runs in 5 min bursts, with 5 mins easy running in between each burst, for a total of 30-50mins with 10 mins warm up. its worth remembering that the aim of a tempo run is to be exercising just below your lactic threshold during the quick periods.