If possible, place it the same distance behind you that the center channel is in front of you. That is a good rule of thumb. If you dont have much space behind you, (less than 2 feet) you might end up just wanting to ditch it.
My last HT rig was in a room about 12ft by 26 feet. The seating position in the center, with the two rear surrounds the same distance behind and apart as the fronts. they felt pretty natural at that distance.
The delay functions can help if you cannot get exact distance between sweet spot and each speaker, but ive never had too good of an experience messing with the delays. If you only have 1-2 feet behind you, you will probably feel that the rear sjust sounds too close.
If your couch is pushed up against the back wall, then forget about the rear surround, you need some space or it will feel gimmicky.