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?