When the impact velocity drops below the functional velocity of the bullet/cartridge (meaning the bullet will no longer expand) I’m using, it’s too far.
When the above happens or when conditions are bad enough that I'm not extremely confident that I can make the shot, whichever comes first, it's too far. The former can be calculated using ballistic software, but the latter can only be determined through extensive practice in various different field conditions.
There are enough variables involved with each shot in the field that I have to make the judgement of what is "too far" for each opportunity that comes my way. Sometimes this is 1000 meters, sometimes it's 300.
