How much time do you spend studying as A2 students?

Highly dependent on which subjects you are taking.
If it's Maths or Sciences, you can generally get by just doing the homework set from what I gather. Obviously consolidating content learned throughout the year will stand you in good stead when it comes to revising, and reading off syllabus can't harm, but it won't really make a difference to how you fare in your A2 exams.

The arts, on the other hand, require much more work outside of lesson and homework time. I'm doing History English and Latin for my A2s, and whenever I am not officially set homework but have some spare time, I am always trying to read up more on the History topic I am doing, or get ahead on the Latin translation, or just reading in general for English (as I intend to study this at University). I got As at AS and am predicted A*s at A2 so I guess what I'm doing is what you should try and do if you have similar aspirations.

Also remember though that it's important to rest! :P Don't work 24/7 and burn out, but yeah, it's more work than at AS.

Hope this helps in any way