。
She's supposed to be back by lunchtime.
Literal
She [topic-は] lunch [possessive-の] time [by-までには] return-come [arrangement-ことになってます].
~ことになってます uses the casual contraction ~てます (dropping the い from ~ています), standard in everyday polite speech. ~ことになっている expresses an externally established arrangement — she's expected back by lunchtime, and this is an agreed-upon or scheduled plan.