Future Perfect Continuous

The future perfect continuous is used to express the duration of an action up to a future point in time. This tense is not commonly used in English.

This tense is often used with the following time expressions:
... by / ... by the time ...


+ Basic Construction:

- Positive:
Subject + will + have + been + verb + ing + object(s)

- Expression:
"We will have been studying for two hours by the time he arrives."

- Negative:
Subject + will not (won't) + have + been + verb + ing + object(s) + time Expression

"He won't have been working long by two o'clock."

- Question:
(Question Word) + will + subject + have + been + verb + ing + object(s) + time Expression

"How long will you have been working on that project by the time he arrives?"


http://esl.about.com/od/grammarstructures/ig/Tenses-Chart/futureperfectcont.htm