만 is used after a period of time to mean "that much time since" or "It's been that much time since ...".
- 고등 학교 때 친구를 5년 만에 처음 만났다: I met my high school buddy for the first time in 5 years.
- 그는 호주로 유학을 갔다가 3년 만에 돌아왔다: He returned after 3 years of studying in Australia.
- 도착한 지 두 시간 만에 떠났다: (He/It) left two hours after arrival.
"몇달만이야 이게" is "몇 달 만이야, 이게?", strictly speaking.
Its normal ordering is 이게 몇 달 만이야? - Wow, how many months (몇 달) has it(이게) been (만이야?).
이게 (this) means "this occasion", or "this particular encounter".
People say it when they meet someone after losing touch with them for a long time.
- 야, 이게 얼마 만이냐?: Gee, it seems to have been ages (since we last met).
- 야, 오랜만이다: Wow, it's been a long time.