Greetings to everyone!
Say your ISP has given you 1Gb download limit per month. Now I want to know how much you have used this month. Would it be correct to ask,
How much of the download limit you have used this month?
If not, what is?
I can't see problems in your sentence! I think it is ok.

Maybe the answer can be 70% of the limit.
I would rather say, How much of your download limit HAVE you used this month?

This is a question, therefore you need to put the predicate before the subject.