Would どのくらい夢がありましたか。be correct? I don't really know. This is for a Japanese assignment I have at school.
どのくらい夢がありましたか may be interpreted as "How many dreams did you have?" rather than “How long have you had this dream?”. I think "どのくらいこの夢を持ち続けていますか?" is more an appropriate translation for “How long have you had this dream?”.
Sorry, I didn't understand (I am not English native) if you by "dream" in this case meant:
1) An aspiration / "target that is difficult to achieve"
2) Something that you "see" when you are asleep (and, if this was the case, one which is repeating, ie the person is asking when you for the first time saw the reappearing dream / nightmare)
Luckily ゆめ works for both,but,
a) If not knowing which one, I would say "その夢はいつからですか" (to avoid choosing between "いつから見てます"/"初めていつ見ましたか" and "いつからもってますか"
b) If I new it was 1), I would say "その夢はいつからもっていますか"
c) If I knew it was 2), I "その夢を初めて見たのはいつ(ごろ)ですか"
I would add この (this) - どのくらいこの夢がありましたか. In Japanese, a lot of meaning is implied and not as specific as in English, however, if you want to be specific this should do the trick.
@Tchibi-kun's basic structure in his answer above with この夢をは、どのくらい有られていったね? can used and is used a lot over here in Japan. First, state the object of your question to be specific, then ask the question. Extremely common here. Don't need the を though. can say:
and other similar options ( these are "spoken" options, don't always play as well in written form ).
I'd risk to use special secret Japanese trick.
It is passive past continuos.