This form of the future perfect tense does not occur very often. It stresses
duration of a single event in the future that take place before another future event.
1. He will have been
sleeping for two days when he wakes up tomorrow morning.
(When he wake up tomorrow morning, he will be sleeping.)
2. We shall have been
living here for five years by next month.
3. By the end of this month
he will have been teaching for twenty years.
4. It will have been
raining for a week if it does not stop raining tomorrow.
the sentence that has frequency can't use future
1. By the end of this month
he will have been climbing ten mountains.
Reverse: By the end of this month he
will have climbed ten mountains.