It depends on the room and how your setup is arranged. If your subwoofer is placed for optimal frequency response (FR), 60 - 80 Hz should be good. In addition to the FR, if you cant tell that your subwoofer is sounding even though you can feel it working, you have done your job right.

If not, you will have to experiment. The aim is to get the flattest possible FR from 15 - 20,000 Hz.

Behringer 1030A, SVS PB-12 Ultra, Etymotic ER-6i
PS3, SGT 10.1, Sansa Clip
Auralex