Over here the test is in two parts.
You have to pass a theory exam (which she did first time) before you are allowed to sit your practical. If you don't pass the practical within something like 2 years you have to sit the theory again. (Both carry a fee of about £50)
There is at present no minimum number of formal lessons, though I believe there is a proposal to bring one in. Your instructor has discretion to decide when you are ready. (Lessons average £20 a time)
The practical test allows you a certain number of 'minor' errors, but one rated 'major' or too many 'minors' and you fail.
It sometimes seems as if it depends what mood the examiner wakes up in as to whether something counts as minor or major!
Fingers crossed now for our son Robert, who takes his test for the second time on the 26th Jan.