Niels' photo does show the infinity focus distance with the 12mm extension tube. It's a bit under 9 inches from the sensor, less than 3 inches from the front of the lens. Your subject needs to be pretty small at that distance. A 35 wouldn't produce a very flattering portrait from close distances either.
The 12mm extension tube on an 85mm reduces the infinity focus to about 2 feet and the subject still needs to be smaller than a human head - maybe eyes, nose, and mouth.
The 12mm extension tube on a 135mm reduces the infinity focus to about 7 feet and you would be able to shoot a head shot at that distance. Of course you could take a similar photo a few steps back without the tube.
So the real question is, what are your trying to achieve?