Buddy and I watched a bull on an opposite hillside from a mile away this past week. Could still hear him when he disappeared into the dark timber. I think there are a lot of variables to answer this question! All depends on the topography in my opinion.
Topography, temp, wind speed, wind direction, direction bull is facing, how loud the bugle actually is (some are booming and carry very far and some are soft and subtle), and more... So many things factor into an "it depends" answer.