I managed about two hours a night but it wasn't always quality work. I did feel quite tired while I was doing it but luckily I had the weekends too so actually went back into the office for most of the day Saturday and Sunday where it was nice and quiet! I used my holidays to attend the "brush up" courses and do the exams and it took me about 15 months to do the lot, although I didn't push it and sat the exams over (I think) 3 sittings to make life easier.
I remember thinking that if I was doing them full time it would have been so much easier - I covered so much in my two weeks "brush up" and got so much out of discussions with the other guys on the course.
Do them full time if you can, but provided you keep focused part time is fine provided you dump as much other stuff out of your life as you can while you're studying!
Good luck!