Indicates the duration of an action which has begun and will continue in future
When Alice finishes his studies he will have been teaching at an university for two 2 years.
subject + will/shall have +been + v1st form + ing + object + for/since
I shall not have been participating in the competition since Tuesday
Will/shall + subject + have been + v1st form + ing + object + for/since
Will he have been writing letters since 2000?
Subject + will/shall not have +been + v1st form + ing + object + for/since
By next June we shall have been living here for ten years