Aria
Why does she put "that" in "for a lot of time"?

The word "that" is used her to refer specifically to the duration of time where she is on the plane. If you were to say "for a lot of time, Siti is on her feet", this would mean that in general, Siti spends a lot of time walking around on her feet. But to say, "for a lot of that time, Siti is on her feet" implies that she is on her feet a lot during the time she spends specifically on the plane.