hey mahvash,
I remember when I was learning the inventions, I wanted to go through pieces faster! But instead, my teacher was very thorough and intent on helping me make MUSIC not learn notes, so I learned all the inventions and took about 5-6 weeks on each invention, given the difficulty of the inventions. Also, as I learned the form and expression of the music (bringing out the subjects, and so on) I learned the music faster, maybe about an invention per 3-4 weeks.
Also, though I thought it was boring, then, it will be of use learning all the specifics of Bach. Bach bored me, because notes are not particularly hard technically, compared to Beethoven or Chopin, but his intentions on the inventions were not so that you can play notes, but that you can make music, and more specifically, for the purpose of instructing students how to COMPOSE.
Anyway, my advice is unless your teacher is making you learn an invention per year or something ridiculous like that, listen to his/her advice, because I'm sure it's worth it later on! Learning repertoire isn't important, imo, just for the sake of "going to the next invention" but instead try to consider what you can learn from each individual invention.