。
She finished her homework in one hour.
Literal
She [topic-は] homework [object-を] one-hour [in-で] finished.
The particle で after a time duration means 'in (that amount of time)' — １時間で = 'in one hour.' This is distinct from に (at a specific point in time) and から/まで (from/until). The duration で answers the question 'how long did it take?' rather than 'when did it happen?' A clean, practical example of this common particle use.