Aaah! Thank you Toritoribe! This is why I couldn't find any English grammar sites on "どんのぐらい"...
So a) I can say - 今日、彼は日本語どのぐらい勉強していますか. Would this go for any similar structure, so I can use this formula - [time period], [pronoun/name は] [noun]どのぐらい[verb]? e.g. [明日],[ベンさんは][テニス]どのぐらい[しましたか]. Then, if I've understood you answer to b correctly, I can move these around a bit, so [time period], [noun]どのぐらい[pronoun/name は] [verb] would work just as well?
b) Sorry I didn't quite understand - I can reorder the words in the sentence and the object remains the same? 今日、日本語, 彼はどのぐらい勉強していますか - Although in this version would I still omit wo? Is this because the subject is obvious from the context?