I believe it's supposed to be about 30 hours worth of lessons but it took me 8 years! I failed my test 3 times between the ages of 18-19 and didn't feel it was right for me to try again until I was 26. I had 6 months of lessons and passed first time then (about 24-30 hours of lessons)
It depends on the instructor, your attitude to driving and how confident you are. You definitely need the confidence to drive. That's what I was lacking at 18. Now, although I don't enjoy driving, I'm glad I did it as I have so much more freedom and don't have to rely on others for lifts.
I passed first time at 17, 4 months after my birthday, after about 12 lessons.
My sister has only just started learning - she's 29 now. She had about 20-25 lessons in the space of a couple of months, but failed her first test, waiting on date for another now.
The avarage learner (if there is such a thing) will take according to the DSA 45 1hr lessons and private practice to pass todays driving test. If the lessons are taken in 2hr sessions you will find you can reduce that to possibably 36 hrs. Its tough out there on the roads now. These figures are taken from the 1000's that pass thier tests each year. The most common reason for failiure is insufficient lessons.
I passed in April this year, think I had around 34 lessons, after about 25 lessons I could drive on my own and my instructor said I was good enough to put in for my test but I still wasn't 100% sure so I had a few more and after them I really felt ready and confident. I guess you have as many as you feel you need. Everyone learns at different paces.