Here is a document with page highlighted explaining how far there is an analysis for terrain after which point, there could be a tall mountain...like Calgary or Denver don't go straight out forever. Look at Fig. 5-2-1
https://books.google.ca/books?id=x8j...0miles&f=false
or google faa ocs 25/46 miles to get to the exact page on the appropriate link provided.
(Note: The AIM can be linked somewhat more easily at https://www.faa.gov/air_traffic/publ.../media/aim.pdf The book reference is at page 5.2.7 - JT)