한두 시간이면 끝나니까 기다려 주세요.
Is it 이다 in 이면 in this sentence? If not what is that 이? I know what the sentence means just curious about this 이 that I couldn't find in Naver dictionary. I also found some website saying it's N(이)면 but without explanation what 이 is.
Yes, your intuition is correct; this is 이다 + ~(으)면.
한두 시간 - one or two hours
이면 - if
끝나니까 - because (something will be) finished
기다려 주세요 - please wait
(Something will) be finished in one or two hours so please wait (until then).
(이)면 can be used to mark a noun as a condition for something. Here, the condition of 끝나다 (to be finished) is 한두 시간 (one or two hours).
You could also say 한두 시간 후면 끝나니까 기다려 주세요 or 한두 시간 후에는 끝나니까 기다려 주세요 and the meaning wouldn't change a noticeable amount.