As with most questions about stuff like this, it varied at different times and in different air forces. For the RAF in the Battle of Britain, here's what Dowding had to say:

A great deal of discussion took place before and in the early stages of the war as to the best method of harmonisation of the guns of an 8-gun fighter: that is to say the direction, in relation to the longitudinal axis of the aircraft, in which each gun should be pointed in order to get the best results.

There were three schools of thought. One maintained that the lines of fire should be dispersed so that the largest possible “beaten zone” might be formed and one gun (but not more than one) would always be on the target. The second held that the guns should be left parallel and so would always cover an elongated zone corresponding with the vulnerable parts of a bomber (engines, tanks and fuselage). The third demanded concentration of the fire of all guns at a point.

Arguments were produced in favour of all three methods of harmonisation, but in practice it was found that concentration of fire gave the best results. Guns were harmonised so that their lines of fire converged on a point 250 yards distant: fire was therefore effective up to about 500 yards, where the lines of fire had opened out again to their original intervals after crossing at the point of concentration.

I haven't been able to find this in print; it's from a RAF report a year after the battle, and the online text is available here. I've also seen anywhere between 250-300 yards for the USAAF. There are also reports that individual pilot's or aces set the convergence to their liking away from the official value; I imagine that would be entirely possible, but I haven't seen anything explicitly confirming that.