彼女は両親に少なくとも１週間に１度手紙をかきました。

。

She wrote to her parents at least once a week.

Literal

She [topic-は] parents [to-に] at-least one-week [to-に] once letter [object-を] wrote.

少なくとも (すくなくとも, 'at least') sets a minimum frequency. １週間に１度 ('once per week') uses the time-frame に frequency pattern. The polite past かきました signals a completed habitual action.