The future perfect continuous tense is used to describe actions that will have been ongoing for a period of time at a specific point in the future. It emphasizes the duration or continuity of an action up to a certain point.

Structure
The structure of the future perfect continuous tense is:

Subject + will have been + verb(-ing) + object

Usage
1. To show duration of an action in the future  
   This tense is often used to show how long an action has been happening by a certain future time.

   - Example: By next month, she will have been working here for five years.
     - (This means she started working here five years ago, and her employment will continue until at least next month.)

2. To describe ongoing future actions with duration  
   It is used to focus on the continuity of the action, especially to describe a state that continues into the future.

   - Example: When you arrive, they will have been rehearsing for three hours.
     - (This emphasizes that they started rehearsing three hours before the specified time.)

 Examples
- I will have been studying for six hours by the time you get home.
- They will have been living in this city for ten years by next year.
- By 9 p.m., we will have been driving for twelve hours straight.

 Time Expressions Often Used
Common time expressions that accompany this tense include:
- for (a period): for five years, for three hours
- by the time: by the time you arrive
- by + a specific time: by next week, by tomorrow

 Key Points
- Focuses on the duration of an action up to a point in the future.
- Requires the auxiliary verbs will have been followed by a present participle (verb+ing).

The future perfect continuous is not as common as other tenses, but it is particularly useful when you want to emphasize the length of time an action will have been ongoing by a certain point in the future.