。
I wonder when mother will come home.
Literal
Mother [topic-は] when come-home [explanatory-のでしょうか]?
のでしょうか is a stack: の (explanatory nominalizer) + でしょう (conjectural polite copula) + か (question marker). The effect is a softer, more speculative question than a plain いつ帰宅しますか — closer to 'I wonder when...' than a direct 'when will...?' 帰宅する is a noun-する compound that literally means 'return-home-do,' the formal/written alternative to 家に帰る.