Well it depends on the questions context if you consider from axumait kingdom it was at 100ad,if is about a United ethiopias start it is around 1855 (tewodrs times),the current Ethiopia was United at the reign of menilik II at 1889. So based on the Ethiopian current land coverage and cultural coverage it has been less than 150 years,but the abyssinian have been here for a couple thousands years. Hope it helps