Choose the correct phrase to complete the detective's dramatic stakeout notes.

"By the time the suspect finally leaves the building," whispered Detective Miller, "we _______ this boring front door for twelve uninterrupted hours."

The correct answer is will have been watching.

When a sentence features a future time clause starting with "By the time..." and specifies a duration ("for twelve uninterrupted hours"), the Future Perfect Continuous is the best choice to describe an ongoing action.

"Will watch" simply states a future fact without duration, and "had been watching" incorrectly places the action entirely in the past.