In lesson 1.9, we're given three examples of how long it takes someone to do something. There isn't really any explanation as to why に in the first example, を in the second example, and nothing in the third example is used. I'm still having a terrible time understanding when to use を, and when the honorific お is used, but this has separately confused me even more.
どれ ぐらい にほん に たいざい して います か？
How long have you been staying in Japan? / Dore gurai nihon ni taizai shite imasu ka?
どれ ぐらい ピアノ を れんしゅう して います か？
How long have you been practicing piano? / Dore gurai piano o renshū shite imasu ka?
どれ ぐらい りょこう して います か？
How long have you been traveling? / Dore gurai ryokō shite imasu ka?
Is thing something that is explained in detail in a further lesson?