The future perfect continuous (or future perfect progressive) describes a continuous action that will be completed at some point in the future.
The future perfect continuous is formed by combining will have been with the present participle:
She will have been teaching for ten years.
We will have been traveling for six months at that point.
Simply add not:
We will not have been fasting very long yet.
You can employ the regular form, although this usage is rare:
Will he have been crawling by that time?
They will have been skiing for two hours.
I will have been waiting for a while.
For more information, please see our introduction to all twelve verb tenses in English.