This takes a bit more mind bending to understand:
몇 시에: at what time
일할: will work
예정이야: is schedule
예정←豫定 (schedule, plan, expectation, preordination)
At what time is your will-work preordination? → What time do you plan to work?
Most Koreans use specific words for each of those translations.
schedule=스케줄 or 시간표 (lit. time table)
and if you're going to use the english preordination it's less awkward if used as preordained which would be 예정하다.
So in this -ㄹ 것입니다/거야 structure, 것/거 can be replaced by a noun?
것/거 is literally "thing" so whatever that thing is can replace it and vice versa.
너는 몇 시에 일할 예정이야 or 너 몇 시에 일할 예정이야 is better.
"what time are you scheduled to work?" This is also correct. We don't really plan to work