www.adeptdrive.co.uk It depends on how many hours experience you have previously had,
for a complete novice aged between 17-20yrs old the national average is 40-50hrs on road experience this can be a combination of private practice and driving lessons.
These figures can be verified on the DSA website, If you really want a good chance of passing 1st time then you should target 50hrs, I would do longer than 1hr lessons the best length lesson lesson is 1.5hrs.
The longer lesson means you learn quicker, need less lessons,retain more and more importantly learn to drive at test standard for longer, this will improve your chances of passing 1st time.
The national average pass rate is 43% the 1st time pass rate is 33%, look for a driving school with a high pass rate don't just ask price. Our average pass rate is 60-70% our first time pass rate is 50%+ this is the type of pass rate you should be looking for, this will mean you found a top quality driving school.