What is considered 'extreme ranges' for McCrees fall off damage?

McCrees buff says he keeps the same damage at long ranges but damage gets lower at extreme ranges.

What is considered extreme ranges?

Falloff range for McCree's weapon is 35 to 55 meters, and the damage would fall off from 70 damage at 35 meters to 20 damage at 55 meters. Extreme range probably means beyond 55 meters of range, which is considered the highest possible distance where the damage is at the lowest.

EDIT: McCree's damage falls off starting 20 meters now.

Source for damage and damage falloff numbers: http://overwatch.gamepedia.com/McCree

According to the Overwatch Wiki on Gamepedia, the damage seems to start dropping at 35 meters.

I suggest trying it out in the practice range on training bots to get a feeling for it - there are distance markers on the ground.
From trying this myself, the damage seems to start dropping consistently, starting at about 34 meters.

It is worth noting that the damage falloff is fairly significant - a body shot at 40 meters distance deals 12.5% less damage than a normal shot. As a result, it takes five bullets to kill the training bot at this distance, instead of three.