Distance of 7.1 back surrounds?

Dolby's version http://www.dolby.com/uploadedFiles/Assets/US/Doc/Consumer/Dolby-Home-Theatre-Speaker-Guide-7.1-6-8.pdf

Audyssey's suggestions http://www.audyssey.com/audio-technology/audyssey-dsx-2

There is no recommended distance, just recommended angles (at least ±30° from the centre line, meaning at least 60° apart).

I wouldn't even use rear speakers unless I had at least 4' feet between my ears and speakers.

How much space do you have?
I have 10 feet behind my couch.

My rear surrounds are up at top of wall at ceiling junction (but cathedral ceilings), about 6 ft at closest point to the main listening position on the couch (big l-shaped job at an angle), but the right surround at the far right end of the couch is about the same distance, otherwise other positions on the couch are further.

I have a small room of 10x11 feet. I'm planning to fit in a 7.1 setup in it. The rear speakers will be 2.5 feet behind the main listening position. Is that too close?

I have my rear speakers at 2.5' behind the listening position and sounds okay in my system. Properly set distances and trim levels in the receiver are key for placing speaker in that short of a distance. Ultimately it will be up to you if you like them that close. If you plan on using automatic speaker setup or room correction, such as Audyssey, I've found that the software tends not to like the rear speakers that close to the setup mic. I suggest either doing 1) manual distance and level calibration or 2) skipping the measurements that are made behind the listening position with the automatic setup.