In typical mountain country you can hear them a very long ways. Line-of-sight across open terrain with no barriers in between, I've easily heard them a mile.
A decade ago on my first elk hunt we cow called and got a bull bugling from almost half a mile away...so he heard that cow call. Couldn't quite get him that day. Came back the next, bugled from the same spot. VERY faintly in the wind, I heard a response. I told my partner I heard him from over the mountain (at least 1200 yards if not more, with a ridgetop between us), and he told me I was nuts. I said let's walk forward a bit more, wait 5 minutes, and bugle again. We did, and this time he was loud and clear at ~800 yds, having come over the ridge top towards our call. We were pinned down and I was afraid to move without spooking him, so I told my buddy to keep calling and eventually my rangefinder could get a read and I nailed him at 638 yards.