Hiya, I'm a year 11 student too, and I'm about to sit my exams in less than a month! I've been experiencing serious procrastination recently, so I haven't been doing much revision either. I've checked with some websites, and the ideal time you should start revising for an exam is 6-20 weeks before.
I don't think it's too late for you to achieve A/A* results. You've said that you've achieved similar grades in your previous exams without really trying, so I can't think of a reason as to why you'd fail these ones. As long as you start revising now (and I mean NOW), I think you'll be fine.
Best of luck to your exams!!