Translation:How long will it take?
As Andrew-Lin says above it's not common to see these two kanji used for these words at all. He's not saying that it's uncommon for them to be used in this particular situation or that these kanji are used for these words but not very often - he's saying that it's more common to see these words in kana - not kanji.
Yeah I feel like what JohnPMChappell does is just type those sentences in the IME editor and click the spacebar so that it turns every word into a kanji, even those words that are commonly written in kana by japanese people. I guess it's cool as a trivia but actually learning EVERY kanji right away is kinda too overwhelming for a beginner and it will get confusing fast.
くらい in kana form means 'about/approximately' whereas in the Kanji form 暗い it means 'dark'. The two terms are kinda related in the sense that when you say 'about', you're 'in the dark' as regards to the exact amount/degree of something.
This is where the Kanji shows its potency and why we should not substitute kana with Kanji in a willy-nilly way when the usage only calls for all kana text.
Are you sure about that? I was taught かかる can be used with both time and money. A search on Goo dict has the definition:
A search on Tatoeba also has examples that clearly use かかる in the context of monetary cost. E.g.:
"どれくらいかかるでしょう" in the first sentence is almost identical to the usage in the Duolingo sentence apart from Duolingo using the -ます verb ending and the か particle instead of でしょう.
どのくらい どれくらい どれぐらい
They all mean how long. Pick your poison.
George from Japanese From Zero says there are specific situations where you would use each one, and that if you scoured the internet, you would learn about those situations. However, it doesn't matter, as the Japanese use them interchangeably. Just pick the one you like and stick to it, or change it up whenever you feel like it.
Nothing in particular. It could just as well mean "How much will it cost?", and that's another common meaning for the same phrase.
どれくらい can be used for stuff like "how long", but also "how far" or "how much".
The verb かかる is rather flexible as well, see: http://jisho.org/search/%E3%81%8B%E3%81%8B%E3%82%8B
Basically the only way you'd know was context, which duolingo is often painfully short on.
See some example sentences here:
That would not work, even with あなた in it. This just means "about how much [time] does it take?". You can't replace the 'it' with 'you' here, as that would sound like the person you are speaking to is a task or some event that requires completion. As is, in the right context, you might be able to translate this as "How long will it take you?", but Duo isn't that flexible (yet, and probably shouldn't).
Tbh, "How long will you be?" doesn't even sound quite right in English. It's a pretty colloquial way to ask how long someone will be away/gone, right? (like asking "When will you be back?") That'd be phrased differently in Japanese.
Everyone is confident here that くらい means "about" and that's what the DuoLingo explanation says. Looking at dictionaries and I'm pretty confident it means something more like "to the extent." Thinking of this sentence as "to what extent will it take time" gives a much better explanation of why "くらい" can't be dropped, which it logically should be if it just means "about".
The basic meaning of くらい (< くらゐ) , as a noun, is "seat" such as "throne" or where the nobility sat, and thus "rank." The last is where the "extent, level, about" sense of it as an adverbializer comes from. But it couldn't possibly be dropped from どれくらい unless it was replaced by something else, like ほど. ほど is also an "extent" word. Either way, "about" isn't necessary in the translation.
English requires a weird way to make yes-and-no questions in the present and past tense. This is called "do-support" (https://en.wikipedia.org/wiki/Do-support#In_questions). It uses the verb "to do", so it would be: "How long does it take?", and the past "How long did it take?" instead of "how long it took?", which sounds non-native!
あれだけのコインを 収集するのに どれだけの年月が かかりましたか。
あれだけのコインを しゅうしゅうするのに どれだけのとしつきが かかりましたか。
(that many coins) (in order to collect) (how much time) (did it take)
How long did it take you to collect so many coins?