if one steel takes 56 hours (56 in memory).
And if this supposed length of making is 30 days times a day, which is is 24 hours, you get 720 hours.
if you divide 720 by 56, you get 12. A B12 takes 12 steel to make.
You made a B12.
Wait, I must be very much wrong, coz my answer doesn't seem related to the riddle in question.
Edit: Okay, let's say this length of making was 28 days... That makes a perfect 12...
Edit 2: Actually, my answer does relate to the riddle. "Sheer power", the B12 is the strongest weapon in the game, no?