。
She's supposed to be back by lunchtime.
Literal
She [topic-は] lunch-time [by-までには] return-come [arrangement-ことになっている].
~ことになっている expresses an externally established arrangement or rule — 'it has been decided/arranged that.' Unlike ~ことにしている (self-imposed habit), this implies the arrangement comes from outside: a schedule, a promise, or someone else's decision. までには ('by') sets the deadline with contrastive は.