I always just "ball park" it as s strong bugle in heavy timber is probably within 500 yards.
If we start talking about across drainages and "distant" bugles, I'll figure 1/2 mile.
Open ground, can be further than that and it depends on how clear, but a clear but distant bugle, for my ears, is probably within a mile or so.
It depends is probably the "right" answer, but those are the rule of thumb I default to for my hearing. And those were developed over 20 years plus of hunting. They come into play when i face a bugle and draw a line on my OnX toward the bugle. I'll draw a straight line further than the distances above and try to figure where it came from. Further bugles I plot as I move. IF he's bugling from the same place it almost serves as triangulation as I close in to figure out where in the terrain he is. If he's moving, it can sometimes show a direction of travel if he goes quiet.
But it depends is probably the correct answer here.