It really depends on three factors: barometric pressure/humidity (density of the air), elevation, and vegetation.

Hence, that's about 9 different scenarios, and hence 9 different answers

For extremes:

1) on a ridgetop, on a clear morning, before the woods green up---miles.

2) in a valley, slightly misting, late spring with thick vegetation---50 to 100 yards.

Of course, the other problem that is often encountered, is scenario (1) typically is the result of a high pressure system and the birds usually gobble their fool heads off, whereas in (2) the low pressure system really shuts down the birds, so there is also fewer gobbles to be heard. Wind can also be a factor, but usually only affects the ability to hear gobblers in extreme cases (30+ mph), which also has a huge impact on bird behavior anyway...