Теория:

Study information about the future perfect continuous.
  
future_perfect_progressive_tense.png
  
We use this tense:
 
1. When we want to emphasise the duration of an action up to a certain time in the future.
 
By the time Tim retires, he will have been working for the same company for 20 years.
 
 
2. With by....for
 
By the time he finishes work, we will have been waiting for more than an hour.
  
There are different types of sentences.
Positive Subject+ will have been+Ving
NegativeSubject+will not have been+Ving
QuestionsWill+ subject +have been+Ving?
 
 
Источники:
www.grammar-monster.com