I agree with Jeffree...it depends on the person. That's the answer to pretty much every question on this site at the moment (that doesn't revolve around "what colour hair do you have").
Also, anyone still umming and ah-ing about finishing a subject before school starts at this point in time is going to have to make up their mind pretty quick to get it done by Feb!
Personally, I worked ahead in some subjects and didn't in others. I got 50s in the subjects I did pretty much nothing for in the holidays, and didn't do as well in the ones that I worked on over Summer. However, correlation does not imply causation! The ones that I worked on over the holidays were my weaker subjects anyway - which is the reason I did a bit of holiday work on them. There's no one-size-fits-all solution: it depends on the subjects; it depends on the person.
I think the key is not to ever think that you have "finished" a subject before school starts. I did all the methods chapter reviews in the holidays but there is no way that I actually completed the course...I just read the theory and did the questions with the textbook in front of me, and asked (on this site) about all the ones I didn't understand. I only spent a few days doing this, and it was only beneficial for me because I had done a lot of the 3/4 topics in Year 11 (didn't do methods, did another maths course). I'm glad I did it because when the topics came up in class, it was a matter of revising and relearning rather than starting from scratch.
On the other hand, I tried (and failed) to read my entire chem textbook in the holidays. Complete waste of time: retention = zero. All it achieved was to make me think chemistry was boring and hard, and to help me fall asleep at night in under three minutes (NB: reading textbooks in bed if you don't understand them is a pointless exercise.) In hindsight, that time would've been better spent reading the Twighlight series since that's what I did in the lead-up to midyears!
For my humanities and art subjects, I didn't really do work as such in the holidays...but seriously, the stuff that you probably do anyway, like watching movies and reading books, helps just as much. That said, the next person might tell you that they wrote 8 essays a day and breezed through the year as a result: it. depends. on. the. person.
I think that one of the things you need to do is trust yourself to do what feels right. Again, this varies between people and depends on the extent of your self-control, but it's pretty much what I did and it worked for me. If you feel like going out, go out. If you feel like slobbing around, be a slob. It's probably good for you. (Note: does not apply to times you feel like eating 4L tub of triple chocolate ice cream). During the holidays this is especially true - holidays are holidays, do whatever you want!
Good luck, and don't worry about how much work you have or haven't done.
More importantly: don't worry about how much work other people have done, especially not at this point.
Have fun this Summer!