。
My mother comes to stay once a month.
Literal
Mother [topic-は] month [per-に] once [topic-は] home [to-に] stay-[purpose-に] comes
Casual register: くる instead of 来てくれる. Without てくれる, the benefactive nuance is absent — it's a neutral statement of fact rather than a noted favor. A useful pair to feel the work that ~てくれる does.