How much work are you expected to do in Sixth form outside of lessons?

My daughter recently finished her A-levels, and I would say she put in about 3-4 hrs each day after school (and there was already some revision/homework time during the school day), plus maybe half a day over the weekend, plus of course extra revision time leading up to the exams. So you can probably figure minimum 20 hrs per week outside school, plus any extracurricular work like special projects or school clubs or work experience, etc.

Sixth form is quite structured in that they give you a timetable and generally expect you to follow the programme the school sets out, but on the other hand it's independent in that you need to take responsibility for actively participating in things and organising your work outside school hours. So in that sense it's sort of a half-way-house between school and university.

A few hours a night from what I remember. How much time and effort you put in is up to you though as it is very independent. You will probably have free periods during the day, so you can use them to work and can keep your evenings relatively free!