Translation:It takes one week.
I am not sure, but I suspect it's because "week" doesn't change pronounciation when you start counting them.
With days, each amount/length has its own word. E.g. 二日 (ふつか), on its own, means "a period of 2 days". There's no need to add かん. You can use the same kanji with days, but only if you include の, as in: 二日の間 (ふつかのあいだ) "a span of 2 days".
間 (かん) indicates a length of time, a certain period.
It's made up of 'sun' 日 (ひ) inside a 'gate' 門 (もん), which I find helps to remember it. It's like a measurement of time; "the period during which the sun is visible through the gate".
On its own, it reads あいだ, and means "interval", "during", or "within". And since time and space are so similar, あいだ can actually mean "between" (physically) as well.
It's pronounced: いっ which is a variation of いち. Note that the small っ denotes a 促音 (そくおん/sokuon) - it essentially tells you that the consonant that follows it is elongated (germinate consonant). In practice, you will hear a very brief pause between い and しゅう, while the mouth position of し is held before ゅう is pronounced. This 'consonant germination' only occurs to voiceless consonants (/s/,/t/,/p/,/ɕ/,/k/ and etc).
Me. I also made a comment about this:
I instantly knew what 一週間 meant because of the anime 一週間フレンズ (one week friends)
I think it is a great anime. I get why not many people are talking about it because it isn't to everyone's liking, but I think they do a great job at exploring the topics of bullying, friendship, perseverance, and a lot more. It even made me think a bit more about the meaning of life. But I'm not getting more philosophical than that today xD
Also, one little note: 一週間 is written as いっしゅうかん and romanized as "Isshūkan" or "Isshuukan". The way you wrote it makes it like a cobination of 一週間 (isshuukan) and 異種間 (Ishukan). 異種間 means "cross-species" :)