The Future Perfect Continuous is normally used to predict the length of an activity — as if looking back at such activity from some finished time in the future:

 

By July, Cindy will have been working in this office for a year (‘for a year’ is an imagined finished time in the future, but Cindy will continue her work after it).

To make the Future Perfect Continuous, use:

 

Will have been + the ‘-ing’ form of the verb.

 

Singular	Plural
I will have been doing You’ll have been doing He/she/it’ll have been doing	We’ll have been doing You’ll have been doing They’ll have been doing

 

By the time I arrive home, I will have been driving for six hours.

 

By this time next month, I’ll have been studying English for a year.

 

Melissa will have been cooking all day long before the wedding cake is ready.