For me, my period of "burn out" happened after the Yr 11 Midyear exams. I had two year 12 subjects, of which i put in a lot of effort into Literature (extra research, lots of writing/editing/re-writing...) and then after a brief cramming session for my midyears - i felt all that motivation go POOF! So for the holidays and ALL of Term 3, i felt tired, and seemed to lack any self-encouragement to keep up with work at all. I took home all my work, then looked at it, said "Screw this" and promptly went to sleep.
So the moral of this tale is, as everyone has put it, space yourself out. It's also natural to feel down/unmotivated/"I-dun-care-anymore" but it's better to break this period of "burn out" quickly or it could become a bad cycle and drag on for as long as a whole term.
Make some kind of pact with your mates so that if any or you enter this stage of almost semi-depression, the others will take you for a night out, etc - a Wake Up call.