Distance from the front not the rear wall?

You need a minimum of 4 feet and 8 feet is a lot better between the listener and any walls. After about 8 feet it really does not matter that much (diminishing returns).

If you cannot get at least 4 feet between you and the wall behind your head then you can reduce the problematic reflections by putting up some RPG skylines or heavy curtains.

Do not under estimate how bad teh wall behind your head can sound. It can ruin the bass and it will totally ruin imaging and soundstage - the sound will be claustrophobic and fatiguing. Those who cannot hear the deleterious effects of walls near the listening position should seriously consider whether it is even worth investing in high end gear.

The simple explanation is that

1) Bass response is always the worst close to a wall - just try it by walking around the room - you always get a more uneven bass response when you sit near a wall.
2) The secondary reflections will collapse the soundstage to the speakers - just try it by moving your position out into the room and you will find the soundstage grows in height and width and frees itself from the speakers. However, with the back of your head against a wall the sound collapses to the speakers.

Depending on your setup this will be more or less apparent. You need to have the speakers free from nearby reflections to begin with to hear 2.

The problem with Sonics explanation above is this statement:-

"Secondly, the reflections are shorter than the circumference of the head, so the brain cannot measure the time delay between the ears, and therefore cannot localize the source of sound. When the brain cannot localize reflections it ignores them."

This is obviously not true for a listener seated at 1 to 3 feet from the wall behind them, as suggested in the article). Physically the reflected sound has to travel at least two feet further than the direct sound when the listener is 1 foot from the wall. As the article mentions at the beginning, this is very bad because of the 5 msec rule which requires that NO reflections reach the ear prior to 5 msec.

If you apply the 5 Msec rule(which is scientifically proven) then you need a minimum 4 feet from the speakers to the side walls and at least 4 feet distance between you and the rear wall behind your head (as I recommended).

Why do I say 4 feet and not 5 feet (since 5 feet is about 5 msec)?

This is because the diagonal path that reflected sound follows will almost always be longer than the physical distance because the sound has to "reflect". You can, in some situations, get away with 3 feet.

The inconsistencies in the article by Joachim Gerhard should be self evident - although a lot of what he says about speaker placement is valid. Although, such a wide speaker separation tends to exaggerate the stereo effect often to the detriment of a more natural presentation in favor of a more impressive presentation.

I think it is very hard to generalize about this stuff.

If everyone accepts that speakers are best kept away from reflective surfaces by several feet (ideally 3 or 4 feet or more in order to get an optimal stereo image and soundstage) then according to physics it is a given that the same applies to the listener. (A reflected sound within 5 msec of that of the direct signal from speaker to listener can be created either way: Speaker=>near speaker side Wall=>listener or Speaker=>near listener rear wall=> listener; and in both cases the imaging will not be as precise as without these close reflections)

I think that the use of specific absorbing room treatment can go a long way in adapting to the situation.

Absolutely.

I have seen research that suggests you need to get the reflections 21 db below the primary signal to make them inaudible (according to lab tests rather than theory).

Diffusion behind the listener might be the most effective. If the listening position is anywhere near a corner then corner bass traps would also help.