Why does she put "that" in "for a lot of time"?
Native speaker’s answer
Rebecca
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.