The future perfect continuous tense is used to indicate a continuing action that will be completed at some specified time in the future. The future perfect continuous tense – future perfect progressive – tense is not used a lot in English grammar, but you will know it when you hear or see it!

future perfect continuous

will +will have been + present participle

be going to + am/is/are + going to have been + present participle

You will have been waiting for more than two hours when her plane finally arrives
You are going to have been waiting for more than two hours when her plane finally arrives.

It is possible to use either “will” or “be going to” to create the future perfect continuous with little or no difference in meaning.
The following sentence is in the future perfect continuous tense:
I will have been studying the English language for three years by the end of this term.
In the above sentence, the future perfect progressive is used to indicate the ongoing nature of the future act of the studying.

We use the future perfect continuous to show that something will continue up until a particular event or time in the future. ‘For three hours’,  ‘for two weeks’, and ‘since last Wednesday’ are all durations which can be used with the future perfect continuous tense. Notice that this is related to the verb tenses: present perfect continuous and the past perfect continuous.  With the future perfect continuous, however, the duration stops at or before a reference point in the future:
They will have been working for over an hour by the time Marla arrives.

This verb tense has two different forms: “will have been doing ” and “be going to have been doing.” And, unlike simple future forms, future perfect continuous forms are usually interchangeable.