"How long will it take?"
いくらお金がかかりますか is ok
You can say in theory いくら時間がかかりますか but it is much more common to use どれくらい for 時間.
いくら～も is common for both time and money though.
- いくら時間があっても足（た）りない (No matter how much time I have, it is not enough)
- いくらお金があっても足りない (No matter how much money I have, it is not enough)
Just as "how" means "to what extent" as in "how are you," "long" means "a large amount in length," "how long" is suddenly talking about the time and not about the physical length?
どれ is which, くらい is extent, so どれくらい is which extent. It talks about "how much" どれくらい必要（ひつよう）ですか - how much do we need.
どれくらいかかりますか = how much (time) will it spend!
A lot of interpretive rather than literal translations are accepted in the Japanese course. I think if you submitted an error report your translation might be accepted, it does mean the same thing, just specifying hours. Using “jikan” would not work if the expected answer is less than an hour, but the English here gives no implication of that.
Why is dore used instead of itsu, if dore means "which" and itsu means "when?" Wouldn't itsugurai make more sense? I'm trying to understand the reasoning behind the word choice when the language was being reformed. I just don't understand how "which" turns into "how much."
To my knowledge, どれ and くらい are colloquially written in katakana.
Technically くらい can be classified as a particle: https://jisho.org/search/kurai%20%23particle
But I think of どれくらい as a whole word by itself: https://jisho.org/search/%E3%81%A9%E3%82%8C%E3%81%8F%E3%82%89%E3%81%84