FUTURE PERFECT CONTINUOUS
Future Perfect Continuous is used to talk about an event that will be in progress for some time before a second moment in the future. The action that came first is the future perfect (or, past of the future). We can identify the future perfect continuous by its structure.
Adding these structures (have + [to be in past participle = been] + verb -ing).
Thus, the structure for past perfect continuous is (auxiliary verb 'have' in future = will have + [to be in past participle = been] + verb -ing):
Will have been + verb in -ING
She will have been taking care of the baby for two days until things go back to normal
If things keep up this way, it will have been raining for three days straight
They will have been working for two days when you arrive.