私は一日中出掛けていた。

。

I was out all day.

Literal

I [topic-は] all-day-long going-out was.

一日中 ('all day long') — 一日 'one day' + 中 'throughout' — is the duration pattern. 出掛けていた uses ~ていた for a continuous state in the past. Structurally minimal; useful as a basic time-duration example.