Is there a difference in meaning between 「何時間寝ますか」 and 「何時に寝ますか」 ?
I know that 「何時に寝ますか。」means "What time do you sleep?" But what about 「何時間寝ますか。」?
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何時間 is a question about a length of time.
何時 is a question about a point in time.
「何時に寝ますか」is indeed 'when do you go to bed?'. 「何時間寝ますか」 (or perhaps a bit more naturally 「何時間眠りますか」) is instead 'how long do you sleep for?'.
(寝る is 'go to bed', 'decide to sleep' rather than just 'sleep'; 眠る is 'fall asleep' or 'be asleep'.)
Time counter words with 間 in them I think universally refer to periods of time, such as 何週間 '(for) how many weeks'; though not all period-of-time counters use 間 (such as 何ヶ月 '(for) how many months').