JammedStab
Your ´question is answered in ICAO Annex 4, Chap. 3.8.1.
Citing from memory, it´s an upward slope of 1,2 % extending normally for 10 km or roughly 5,5 NM. In some cases that surface is extended to 12,5 Km or 6,75 NM.
I´ve done consulting work for an Airport where on one rwy the Aerodrome Obstacle Chart - Type A chart extends for 15,5 km before the sloped surface get clear of the hills.
Jo