ref – https://www.natterandramble.co.uk/future-perfect-continuous-uses-form-timeline/

[will] + [have] + [been] + [present participle]

The future perfect continuous depicts past/present/future actions that continue up until a certain point. Like the future perfect and future continuous, it’s used with a specified time.

sentence: By October, we will have been working on the project for over a year.
explanation: They started working on this project in the past or present.
It will continue up to October, which marks their 1 year.

sentence: In ten minutes, my parents will have been waiting in traffic for four hours.
explanation: Their parents were stuck in traffic. (past) When it continue up to a certain point (ten minutes), they will have waited for 4 hours.