。
I write to my mother once a month.
Literal
Mother [to-に] one-month [per-に] one time letter [obj-を] write (polite).
The に in ひと月に一度 is the 'per' use: 月に一度 = 'once a month,' 日に三回 = 'three times a day.' This is distinct from temporal に ('at [time]'). ひと月 uses the native reading (ひと) rather than Sino-Japanese いち — the native reading is more conversational with 月 here. 手紙を書く is the standard 'write a letter.'