Sentence structure for "I have been ~ for X days"


I'm trying to say something along the lines of "I have been studying Japanese
for 12 days". So far my research has led me to this attempt:

> わたしはじゅうににちにほんごをべんきょうしています。  
>  watashi wa juuni nichi nihingo o benkyou shite imasu.

I suppose my question is whether this is a valid sentence, and if not, how
would I phrase this so that it makes sense?




(私{わたし}は)１２日間{にちかん}、日本語{にほんご}を勉強{べんきょう}しています。  
(watashi wa) juu-ni nichi kan, nihongo wo benkyou shite imasu.

You don't need to add 私/僕/俺 if it's already understood from context that you
are talking about yourself. Apparently it can seem obnoxious.

Numbers are often written as the actual numbers.

If you are just starting, I really recommend you check out Misa's lessons on
YouTube! Here is her beginner
[playlist](https://www.youtube.com/watch?v=GUymRdqBglo&list=PLd5-Wp_4tLqZor0fbsXtP7t6npWeE-2IU).
She does videos up to N3. Watching her made some things click for me the way
reading about them didn't.

