Asks or indicates the extent, limit, or degree of something; 'how far', 'to what extent'.
この計画はどこまで進んでいるの？
How far has this project progressed?
どこまで手伝えばいいですか？
How much should I help?
彼の判断がどこまで正しかったかはまだ分からない。
It's still unclear to what extent his judgment was correct.