Besides will + have + been, the future perfect continuous also contains the verb's present participle (verb root + -ing).
There is a verb tense known as the future perfect continuous, which describes actions that will continue until a certain point.
· Jack will have been waiting here for four hours by six o'clock.
· By 2023 I will have been living in Delhi for sixteen years.
· When I finish this course, I will have been learning English for twenty years.
· Next year I will have been working here for four years.
· When I come at 6:00, will you have been practicing long?