We've never given our helper a curfew and never had any problems. The way I look at it, working for us is her job, just like anyone elses job. Would an office worker be asked to go into the office on their day off (well maybe occassionally, but not on a regular basis)? So we don't ask our helper to come home, to her place of work, early on her day off.
I agree with Cara, helpers are adults and 7.30pm is very early for an adult to be expected to come in. My curfew when I lived with my parents at home, at 18 years old, was midnight. If a helper comes in so late that they can't do their job the next day, then that's a different issue and needs to be addressed in terms of them not being fit for work. Your helper is probably agreeing to the time as she doesn't want to upset you and wants to give you the answer she thinks you want to hear. If you feel that you must have a curfew then I agree with the others who suggested you move it to a later time. Maybe get your helper to suggest a time that she is more comfortable with, then she is more likely to stick to it if she suggested it.
I agree with Cara, helpers are adults and 7.30pm is very early for an adult to be expected to come in. My curfew when I lived with my parents at home, at 18 years old, was midnight. If a helper comes in so late that they can't do their job the next day, then that's a different issue and needs to be addressed in terms of them not being fit for work. Your helper is probably agreeing to the time as she doesn't want to upset you and wants to give you the answer she thinks you want to hear. If you feel that you must have a curfew then I agree with the others who suggested you move it to a later time. Maybe get your helper to suggest a time that she is more comfortable with, then she is more likely to stick to it if she suggested it.