18" is what everyone states, though it's not expressly described in SQF code. They also state you "must" paint it white, which I've never seen written anywhere. The claim for the white is so auditors can look all the way down a wall and note whether there are dark rodent droppings or accumulated dirt.
SQF code makes numerous references to being able to get behind racking and stored material to clean, hence why we have to leave space. You don't have to be 18" specifically, but it needs to be sufficient room for a person to move down the space to clean or access pest traps placed there. Had a chubby auditor once who felt 18" wasn't sufficient because he couldn't fit down the space to go look at a trap, he told us it was going to be a finding. We argued with him about it, and when I (who am also heavy set but less so than he) showed I could easily maneuver down the space to bring him the trap to inspect, he dropped the matter.