It all depends on your ability to understand and master driving, an average learner with no previous experience would require at least 40 - 52 hours of tution before they are ready for the test, all drivers are different when it comes to driving this figure is just a guidline, some learners have taken the test with only 25 - 30 hours of tuition, there are other learners who have needed between 60 - 80 before they have taken the test.
Once you are able to drive independently safely for a period of 40 mins without help from your instructor and don;t commit serious or dangerous faults. Your instructor is satisfied you are ready for the test. However if you have taken an absence from driving for a while get professional advice and training before committing yourself to a driving test.