오렌지를 먹어본지 얼마나 오렌지?
To follow correct spelling, the last word should be 오랜지, and it's not a question ending, so '?' shouldn't be there.
오랜지 is the adjective 오래다, meaning "(it's been) a long time", in -ㄴ지 conjugation. -지 오래다 or -지 오래됐다 means "It's been a long time since ...".
It is an exclamatory sentence, like "How long it has been since I ate an orange!".
The only thing funny is that 오렌지(orange) and 오랜지(오래다 + -ㄴ지) sound roughly the same when spoken, so the last part sounds like repeating 오렌지, creating a crude kind of pun.