It entirely depends on your work ethic.
I was one of those people who, at GCSE level, could get away with doing absolutely no revision, and still get good grades. They were just good though, B's and C's. I was definitely capable of getting A's but I didn't try hard enough.If you're like me, then A levels will be pretty horrible. I don't cope well under the stress of learning because I've never experienced it until I got to college.
If you end up having WJEC as your exam board, good luck to you; for many things, they're the hardest. OCR chemistry is horrible.
One thing I will say, is from day one, revise. Seriously, that can't be stressed enough. If you don't revise, YOU WILL FAIL. The end.Revision is boring as hell, everyone thinks that - because it is. Don't let that be your reason not to revise though, because you'll kick yourself for it on results day.
Make a revision timetable. Revise when you have free slots on your regular timetable, so that there's more free time at home.This sounds excessive, but aim for 5 hours of revision per subject, per week. That's A levels though.
They're very hard work and require pretty much 100% commitment. You will most likely suffer panic attacks, many get depression and other mental illnesses as a result of studying A levels, but there's help available online, here and in your college.I thought it'd be better for you to read all of this than to just say "oh they're awful" or "you'll be fine as long as you revise" because they're only part of the truth.
A levels are probably the hardest part of education, many say harder than uni. You're social life will dwindle in that time. But it's just two years of your education and your life, but they're so important. Make them count!!