Talent-Nature of terrain is the more critical issue;any range which has a marked ridge to the lee can generate severe rotor turbulence.In the U.K. most incidents have involved the Welsh mountains,Scotish Highlands and the Pennines.
-Lenticular clouds can extend up to 100 miles downwind (high level).At low level standing rotors can form up to 20 miles downwind of a ridge.They do not break away but will form in different positions in different windspeed/stability conditions.Light winds and stable conditions produce short wavelengths,stronger winds and less stable conditions produce longer wavelengths.On a day such as this (strong easterly) I had my bumpiest ride ever coming into Manchester on 06.
In the case of rotor streaming the airflow does separate but the turbulent region remains close to its ridge (a few miles).
In both types incidents of severe turbulence have been recorded at very low level (not just at ridge height)