That question is very common, and of course the rate at which people learn is different for every individual. I can give you an honest answer after your first few lessons.
THE DSA RECOMMEND THAT YOU SHOULD HAVE 40 - 45 HOURS OF PROFESSIONAL DRIVING TUITION, AS WELL AS PRIVATE PRACTICE!
The national average according to the DSA is 46 hours of professional tuition and 22 hours of private practice. If you want to pass quickly you might want to consider Intensive Driving Lessons.
It certainly helps to re-enforce what you have learnt during lessons if you are able to go out and practice with a family member of friend. Remember, that person must be over 21, with at least three years driving experience, and you must of course be insured on the car you are practicing in and display legal sized L plates on the front and back of the vehicle. It is helpful to practice with someone who is calm and patient, and who won’t try to teach you their bad habits!
Attempting to take your test when you are not ready is a false economy; it will end up taking you longer to pass in the long run, as well as the added expense of booking tests and hiring the car to use on the test. If you don’t pass a test, try to keep up your lessons before taking another one so you stay at the required standard. A lack of continuity can cause you to become rusty.