I've seen '멀다 하고' used in two sentences as '사흘이 멀다 하고' and '하루가 멀다 하고'. Please could somebody explain to me what the '멀다 하고' means? Can you use it with other periods of time e.g. 이틀 or even hours / weeks etc.? Thanks in advance.
Both of them are idioms. The sentences "하루가 멀다 하고" and "사흘이 멀다 하고" mean someone feel a day(하루) or three days(사흘) is too long to wait for them, so they do something repeatedly frequently. For example,
민수는 하루가 멀다 하고 컴퓨터 게임을 한다.
means Minsu cannot wait for playing computer games, so he cannot help himself from computer games.
Using "멀다 하고" with other periods of time is less usual, but you may use it. For example, you may use it as follows:
십 분이 멀다 하고 화장실에 다녀왔어.
This means the speaker went a restroom so frequently, so the time interval between visiting the restroom could be less than 10 minutes.