I've read someplace at least 4'. I have mine set at approx 8 - 12' depending on where you sit and have been happy with the result. You should also use Audyssey if you have it to help adjust (i also did some manual over-riding with an SPL as well)
I agree with the 3-4 foot range, and more is always better. This typically applies more to freestanding speakers than mounted ones. Having speakers (or seating) too close to walls or corners results in a lot of low-end buildup that can degrade the quality of the sound.