Теория:

Study some information about signal words in Future Perfect Continuous.
 
Words for Future Perfect Continuous:
 
since — с какого момента;
 
when — когда;
 
by the time —  ко времени;
 
by —  к;
 
for — в течение.
 
Examples
 
Mike will have been writing a poem for \(2\) hours.
 
She will have been reading a book for \(3\) hours by the time he comes.