User blog comment:Maki Kuronami/Lesson 28: Humanity did not receive a Grimm reminder/@comment-24792226-20141031111459/@comment-11188061-20141101003825

It can also be interpreted as, "don't reveal it at such a poor battlefield as this, at least choose a more glorious, epic one." Not to mention that I don't a believe an individual use weapon with such a ridiculous recharge time can ever be useful. Remember that Velvet wanted to use it to probably clear out the concentrated Grimm right before her. Numbers wise it was just okay, and this is a city area so it can't be too huge a weapon of destruction (although we do know it's at least AOE).

So it doesn't make sense that it takes a whole semester to 'charge up', the wording is 'build up' anyway. It's far more likely that she is literally building a powerful weapon, and as you can see, with Coco absolutely adoring ultimate weapons of destruction she wanted to give it a more appropriate... baptism.