You don't necessarily have to study each subject each night. Realistically, you'd be spending a minimum of 45-60 minutes on one subject to get some reasonable amount of work done. Considering you have 5 subjects, you can see why this is not really possible.
What I recommend you do is complete any set homework and from there onwards prioritise your subjects according to how soon assessments are. The cumulative effect of doing even a little everyday is significant, but consistency is the key. If you've got this down pat, then I'd move on to focussing on subjects you have difficulty with, clarifying any misunderstandings and doing as much practice as possible (e.g. exam questions, essays etc.).
If you're keen, then I'd recommend going ahead during the holidays; again, especially with subjects you're having trouble with.
Now obviously, this is just my opinion and a generic one at best; your mileage may vary.