As low as you can get and still sound good. You need to factor what the driver/cab can do, what freq your main speakers cut off at, how much efficiency do you loose by going that low, and at what point does the resonace in the room just become a distraction.
Even if you could do a 12 hz tone at 90 db, most rooms would rattle so much it would be annoying and distracting. If your main speakers are only good down to 100 hz (common problem with small drivers), then you better have something that will reproduce 100 hz - 50 hz and not worry so much about the low twenties and teens.
I've seen alot of HT's with dual 4" drivers in the mains that can't do 180-80 hz for shit, and a sub that thumps at 50 hz and I find it annoying because of the huge holes in the spectrum.