。
She worked at that house for twenty years.
Literal
She [topic-は] that house [at-で] 20 years worked.
Working 'at a house' rather than 'at a company' (会社で) implies domestic employment — a maid, housekeeper, nanny, or live-in caretaker. The で marks the location where the work happens. ２０年間 specifies a duration; the 間 ('interval, span') after a number-of-years frames it as a period during which something continues. Without 間, ２０年 would feel more like a point in the past ('twenty years ago') depending on context. Domestic service of this scale once was common in well-off Japanese households; it has receded with social change but the vocabulary endures.