* Wind conditions: Scent travels further with the wind.
* The type and strength of the scent: Stronger smells, like carrion, can be detected from farther away.
* Individual hyena: Some hyenas may have a stronger sense of smell than others.
However, hyenas are known to have a very strong sense of smell, some sources estimate that they can smell carrion from up to 6 miles away.
Keep in mind that this is an estimation, and the actual distance will vary.