Yes, Duo is pretty strict on some things. And your translation, while not usual Eng., had the right meaning. Losing points around here is something we get used to. But repetition is the key to learning, isn't it. Look at it this way: it'll help you polish up your Eng. while learning Italian. If you need any help let me know then when the Gr comes out I'll be asking you and Valentina, ok?
I'm going to drop this here as it seems many people are still confused; the sentence does indeed literally mean "in how much [time] [will] you arrive?", and "by when" isn't the closest meaning in my opinion. Dropping "tempo" is colloquial, as is switching the future with the present, but "fra quanto tempo" is very common in Italian (it is indeed considered a quantity); fra/tra in reference to time indicate a time counted from the present, e.g. "Partirò tra un'ora" means "I will leave an hour from now".
My first answer was "In how long do you arrive?" which sounded stilted to me, but then again sometimes Duolingo seems to require something quite literal and I try not to lose a heart by obliging. Since I could see that one of the translations given for "fra" was "in", I wrote "in how long." For my correction, I tried "How long until you arrive?" Duolingo accepted that although it diverges from the totally literal. I was surprised but pleased to see that sometimes the program adapts itself by accepting translations that are closer to colloquial English.
I said "How soon do you arrive?", but it was not accepted. I will question that. They have accepted many of my "suggestions," by the way, so it pays to check "My answer should be accepted" if you're pretty sure you are right. We are all contributing to the program (but I'd still like my heart back).
Now, look what you did. I'll be singing "I left my heart in S.F. " for the rest of the day. :-) Seriously, I love your analogy. Losing hearts never really bothered me (well maybe at the beginning) but now I'll have reason to smile and go on. I'm sending some lingots to share with S.F.