Depends on the variety and whether you want enough room between it and the wall to be able to trim it twice a year, but a minimum of 2 feet, more if you do want access.

The rule of thumb on hedges is to let the hedge grow half as wide as its planned height. So, a 2 meter tall hedge would need a meter of space, and the bushes should be planted in the center of that space. As Bamboo says, allow another 30 to 50 cm for access to the wall (for painting or pointing). Don't be surprised if the back side of the hedge turns brown and/or bald, due to lack of light and air circulation.