Actually, it isn't. A Google search for "finché arriva" returns hundreds of thousands of results. A lot of them seem to be a famous quote by Einstein, but they appear to be written by native speakers.
You may want to check out the Wiktionary entry on finché: http://en.wiktionary.org/wiki/finch%C3%A9. It's one of those funny expressions that may mean two opposite things, depending on the context. The reason you see it more often followed by a negative verb is that Italians use that construction to express "until." If you think about it, "as long as X doesn't happen" logically means roughly the same thing as "until X happens." And if you know any Italians who speak English, they may say that (somewhat odd) expression in English.
Another great online resource is the Corriere della sera dictionary: http://dizionari.corriere.it/ CDS is probably the most-read newspaper in Italy. I find that the dictionary gives simple, easy-to-ready definitions. If you're interested in learning the "correct" (i.e. Tuscan) pronunciation of vowels, it's also reliable for that. Unfortunately, it doesn't do any "close enough" searches, so you need to make sure you get the spelling and accents right.
The CDS entry for finche' also gives two good examples. In the first (finche' c'e' vita c'e' speranza), it's easy to translate as "as long as." In the second example (lo aspetteremo finche' [non] arrivera') is great example of the slippery nature of finche'. Logically, there should be a "non" in there: We'll wait for him as long as he doesn't arrive ("We wait until he arrives."). However, you may hear native speakers leave the non out without changing the meaning of the sentence (so finche' would mean "until").
Arrrgh, I'm doing this lesson for the third or fourth time and can't pass it because of the mistakes in English translation! When I translate a phrase (not only this one, any) literally, they correct my English; when I translate a phrase so that it sounded "English", they say, it's an incorrect translation. By the way, till and until are synonyms, aren't they?
I finished the Italian tree and had similar issues with articles (where Italian uses them more often) and subject pronouns (where they use them much less). I'd like to see Duolingo do a better job of nudging learners toward forms that may seem a little unnatural at first but are actually much more common in the target language.
Three or four times? Wow, it's amazing. You don't give up. I don't know if it is happened to me. :D
If I have a hard time to understand the question or sentence, I will write it down on a paper (writing on computer doesn't work for me). It will help me to remember when I redo the lesson.