I'm a first year NatSci (similar number of contact hours in term time), and my director of studies told me I should be doing a couple of past papers for each of my four subjects, reading through all my notes from term time, and making sure I know and understand everything, and redoing every piece of supervision work I've been set this term. I also have to do a computing project, for maths and a formal report for physics. I've got mock exams when term starts, but I've not done all this. I don't think many people work as hard as they're told to over this holiday certainly. I can't tell you whether it was a suitable amount of work or not, because I've not done my exams yet. However, I would say don't work too hard - you do so much in term, that you've got to give yourself time to recover.
Also my director of studies is evil... he told me if I had more than an hour a day in term time when I wasn't working, then I wasn't working hard enough. So what I've been told to do probably isn't normal.
Edit: I reckon I'll have averaged 3 hours work a day...but most of it will have been crammed into the last week.