I'd recommend 12-18" as if you have a meltdown issue not related to that wire, you can unplug the fuse without your hands being right on top of the battery. 6" is dangerously close if you have a direct short elsewhere in the car and are smoking the batt.
Always put the fuse holder in first, and leave the fuse out when hooking up the amp. Once everything is wired, insert the fuse. Leaving the power to the amp hot when you are installing leaves room for a slip to blow up amps/speakers and melt parts of the car. Bad idea.