I would have said about 65 yards, which has been my maximum in 50 years, but KRal says he's done 80 yards, sooo...
I use barrel lengths from 5 1/2" to 14", depending upon the gun.I can tell a "significant " difference with the 14". Terminal effect is also significantly different using hollowpoins, but obviously at the expense of meat loss.
With the smaller prairie dogs, about 75 yards has been my "maximum". I intend not to wound game.