Well I would say that it took me a month after passing my test to be totally relaxed in the car, and probably able to walk through another test. Getting used to the roads and getting used to a car are different things. If you are taking your test in another car (which is not a requirement btw) then just drive it for an hour before your test to get used to it.
As for learning the rules of the road you do kind of need an instructor as they can be quite particular. Like a fault in my mock was not putting my right indicator on at a crossroads even though I was in the right lane, and there were traffic lights.
I would say that getting used to driving and following basic rules can be done under provision by parents etc, but driving instructors are there for new information, not practice. Getting a highway code book and looking up anything that puzzles you is a good idea, then you are pretty much your own teacher. Also, go to the area around the tset center and get used to the roads where you will start your test.
Some advice I always give is that when you are on the open road and have time to think, think about the gears and speed limits that are about to come up and always think ahead when you have the time.