"Standard" distance for measuring mechanical accuracy of the gun is 25 yards. That's done with a Ransom Rest, if you have one, or more commonly, sitting at a bench, with your hands resting on a sandbag, trying to eliminate the human factor. (Resting the gun on the sandbag can affect the point of impact as the gun bounces off the rest, and muzzle and cylinder gases can tear up the rest.)
If you are trying to measure marksmanship, instead of mechanical accuracy, then it depends on what you intend to shoot. If you are going to shoot rabbits, you need to be able to determine at what distance you can hit the kill zone of a rabbit. For self-defense, you need to know at what distance you can consistently hit a ten-inch circle (that being a rough estimate of the "stop zone" for a person). Many "combat shooters" can hold 10" at 100 yards, especially using braced positions like kneeling and prone.
If you intend to compete in the sport of Bullseye shooting, those guys work at 50 yards, and try to hit a 3.3" 10-ring, and a 1.7" X-ring. Gulp! Don't get too discouraged. Most people don't shoot all 10's and X's, do they? If you can keep all your shots in 10", then you can go out and have a good time participating in the competitions.