Future perfect continuous:
will have been + V3 / Ved
is used:
1) with how long, for and since for the future
I will have been playing rugby for 10 years next year.
It is most often used with a time expression.
I … (wait) here for three hours by six o’clock.
I will have been waiting here….
By 2017 I … (live) in London for sixteen years.
By 2017 I will have been living in London for sixteen years.
Next year I … (work) here for four years.
Next year I will have been working here for four years.
When I come at 6:00, … (you / practise)long?
When I come at 6:00, will you have been practising long?