Have you heard how many hours she spends studying daily?
ーDid you hear how much she studies ever day?
「彼女が毎日どのくらい勉強するが聞きましたか?」
Did you hear how much ...? のように聞けます。
ーHave you heard how many hours she spends studying daily?
「彼女が毎日何時間勉強に費やすか聞きましたか?」
Have you heard how many hours ... ? のようにも聞けます。
ご参考まで!
Did you ask her how long she studies a day?
Did you ask her how much time she studies in a day?
Did you ask her how many hours she studies per day?
how long / how much time / how many hours
が合います。
今回の自然な言い方は
Did you ask her how long she studies a day?
でOKです。