By the way you ask your question, I'm going to assume you mean for self defense. That being the case, 7yds (21ft) is the standard for max distance you can legally defend yourself with deadly force. Now don't quote me on this, but LEO's are trained to use deadly force when a suspect has a "bladed object" (knife, ax, hatchet, sword, etc.) and is advancing within 30 ft, so I would think the same would apply to civilians. They must be advancing though. That being said, I have a measured distance of 30 feet at my range, and I practice at all sorts of random distances when I'm there. I never measure. Why? What's the point? If you're attacked in a real life situation, are you going to break out a tape measure and measure it off? Learn what 30ft and 21ft look like and just go from there.
Now just for useless knowledge, depending on what state you live in, those distances may not apply in your home. SC has the "castle doctrine" law that gives you a little more legal room for defending your home/property. For example (I had a cop tell me this and have had it confirmed by several others, all from different agencies within the state), if I'm in my house or yard or whatever and I see someone breaking into my car and stealing stuff out, and say I'm 50ft away (well being the usual self defense range), given I'm confident enough in doing so I can take a shot at him and be legally justified in doing so. Now pretty much all of them made it very clear that they did not recommend doing that at all, however legally there is nothing wrong with it. You have a legal right to use deadly force to protect your property in "your castle".