Good question! I remember stuggling with this very measurement myself! The distance is 3 feet, which is the same distance as most hallways. Not alot of room back there, but you do not need alot of room back there. Just enough for one person to manuever around and make drinks - especially is space is at a premium like it was for me.
The picture below is from September 2008, as you can see the space between the wall and the back of the bar is only about 3 feet.
For my kegerator, I went with a Summit Under Counter Kegerator...
As you can see from the picture, I used a PVC pipe wrapped in insulation to keep the beer line running from the nice cool kegerator to the tower. If not insultaed, the first beer of the night poured will be the warm beer that has been sitting in the beer line. Gross and unhealthy. So insulating the beer line as it comes out of the kegerator, through the bartop and into the tower is paramount.
When I put the bartop on, I drilled a hole in the bartop and pulled the PVC pipe (with beer line in it) up through. Then, carefully put the tower on the PVC pipe, I had to trim down the PVC pipe a little, but expected that, as I kept the PVC pipe big and long to make installation easier.
Also, do not buy a stand alone Kegerator because it is only $500 while the under counter kegerator is $1000. It has to do with how each different model disperses the heat. Under the counter has limited space to where the heat goes so it uses a different system then the stand alone. Don't skimp. If your kegerator is going to be built, don't use the stand alone and build it in...it will overheat and cease working destroying the beer inside- the real trajedy.
Oh I forgot...here is a link to the SUMMIT KEGERATOR
And here is the story of my bar build. Each post is numbered because that is how I roll. BAR BUILD STORYEdited by brian12566 - 10/3/13 at 5:45am