There are a few things that you can consider when it comes to thinking about when you should start revising.
To start, there is no set time that you should, no right or wrong date. It all depends on you as an individual.
So, if you can and there is still time, plan early to identify the best time to start revising - consider how many subjects you are taking, how many exams are coming up, when are they all scheduled. If you can figure all that out then you have the basis of a revision timetable. Consider how well you want to do? If you are expecting A's then the longer you have to revise the better.
Get better with time management, while you have your revision timetable, consider what else you have going on in your life because lets be honest, your entire life isn't going to be put on hold for revision. And the best part is, it doesn't have to if you plan properly. Once you consider everything else around you, is there really that much revision opportunities? How much time do you really have?
Once your timetable is complete, calculate backwards. Let's say you're studying ten subjects, and you want to know how much time to put aside per week, you need about one week of revision per subject. That is 10 weeks worth of revision. Of course that isn't 10 weeks solid, that is including the rest of your life commitments. But this is realistic if you want the best grades.
Now you can find your revision start date! But if my guess is correct, you'v got a little bit more time to go.
Hope this helps!