Please explain 要不了多久 Please help me explain how 要不了多久 is translated into "It won't be long"

Literal meaning would be "Could not demand how long?"
I am not sure how 多久 works in this context.
Your confusion came from only thinking 多 as "how" as in: 


多少? (how many?)
多快？(how fast?) 
多久？ (how long?) * time


However, 多 can, and mostly means "many/much" as in:


多日 (many days) 
多人 (many people)
多時 (much time)


In 過不了多久, you don't see a question mark, which mean it is not a question, and 多 here means " much" and 久 means " long time" 

The structure of this phrase is: 
要不了(would not take) 多(much) 久(long time) 

多 here function as "very"