Fighter pilots were taught to use the collimator ring of their gunsight to assess if they were in range for firing their guns.

Although there are written stories of aces claiming they shot down enemy aircraft from distances beyond 500 m (1600 ft) the fact remains that most of the kills were obtained at much shorter ranges.

A study of the VVS (Soviet Air Force) showed that 75% of all of their fighter victories in WWII were in the 50-150 m range (160-500 ft) in the first pass.
The USAAF wrote in the P-47 Training Manual that the best range to fire was from 60-300m (200-1000 ft).
A study of 3600 combat films revealed that 71% of the kills were obtained at 1000 ft or less.

Range estimation was made by comparing the wingspan of the enemy aircraft with the gunsight ring diameter as shown on the infographic.
When the wingspan was 1/3 of the ring diameter your fighter was still too far away. At 1/2 you were well positioned.

The stories of the aces that claim to have shot down enemy fighters at extreme distances must be assessed carefully. Experienced fighter pilots reported that they usually overestimated the range so they may have been closer than thought but it is true that some of these aces did achieve some very long-range kills.

Major Thomas B. McGuire, USAAF's second leading ace during WWII said: "Go in close, and then when you think you are too close, go on in closer".

Major Erich Harmann, Luftwaffe's (and the world's) leading ace said: "I opened fire only when the whole windshield was black with the enemy ... at minimum range".

Lt. Col. John C. Meyer from the USAAF stated: "I am not a good shot. Few of us are. To make up for this I hold my fire until I have a shot of less than 20 degrees deflection and until I am within 300 yards".