You've got plenty of time before your first set of exams, you should be fine as long as you make sure you properly revise once your exams are approaching. Revising all year round isn't necessary at all, although those that do that establish a good work ethic with definitely is a plus. When you've been lazing around all year, it can be tough to suddenly make the transition to properly revising once your exams are coming up. In the end, you know yourself the best, if you're sure that you'll be able to work hard once the time comes, then go ahead and enjoy yourself for now.