I do a minimum of 12 contract hours, but I usually end up doing 16-20. It's hard work doing both, but it's do-able. I work 4.30-8.30 shifts during the week normally, so it's not too bad as I have the rest of the evening free. Just make sure that you've got time off during the actual exam period, I work for Sainsbury's and they've allowed me to have a whole 2 weeks holiday during the exam period.