It’s been ages.
 →   무척오래되었지.

[예문]
W: Mark, when was the last time you went to a concert?
  마크, 마지막으로 콘서트 가 본 게 언제니?

M: It’s been ages. The last one I went to was Art Center in Gangnam.
  무척 오래되었지. 마지막으로 간 게 강남에있는 아트센타였어.

W: How was that?
   어땠어?

M: It was awesome!  
   너무 좋았지.