when do you retake your driving test

There’s a different process in Northern Ireland.
You can drive as soon as your ban is over and you’ve passed the tests you need to take.

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If you fail the second road test, you can pay a fee of $10 that allows you two more opportunities to pass the road test. If you do not pass the road test, you can pay the fee again and take one or two more road tests. If you pass the road test on the first of two tries, you do not receive a refund for the second road test.

After you receive a learner permit for a non-commercial driver license (Class D, Class DJ, Class E, Class M, or Class MJ), you can take two road tests. If you fail your first road test, you can schedule one more road test and you do not pay a fee.

References:

http://www.silversurfers.com/speakers-corner/should-over-80s-take-a-practical-driving-test/
http://nysdmv.custhelp.com/app/answers/detail/a_id/242/~/number-of-road-tests-a-person-with-a-learner-permit-can-fail
http://drive2us.com/intensive-driving-courses/