Though I don't think anybody has exact data on this issue, It seems to hit about up to 2.0 on the icon - 2000 yards. I know because in some occasions, I've been shot down by an Ostwind from 2.1 or so.
But, in most cases, hitting a plane from an ack-mobile is incredibly hard once the range goes over 1.5. I don't know if this is true, but the Ostie 37mm anti-aircraft cannon seems to drop more in recent versions than it did before. I can't seem to hit anything these days(and I used to down a LOOOT of planes with Ostie).
So, a relatively safe range is the visual icon range of 1.5 I'd say.