The Future Perfect Continuous (also called the Future Perfect Progressive), like the rest of the progressive tenses, is used to emphasize the duration of an activity. In this case, the Future Perfect Continuous is used to emphasize the duration of an activity that will be in progress before another time or event in the future. For example, “By 2010, they will have been dating each other for nine years.” “She will have been teaching English as a Second Language for thirty years when she retires.”
This is how it’s constructed:
The Future Perfect Progressive can often be used interchangeably with the Future Perfect with no difference in meaning. This is the confusing part for ESL students. When teaching English language learners this tense, reinforce the fact that it is used when you want to emphasize the duration of the activity that is or will be in progress until a future time or event.
More information on ESL Future Perfect Continuous.