If my driving licence is revoked, how soon can I retake my driving test?

In theory, immediately.  Revocation applies only to drivers who reach or exceed 6 points within the first 2 years of obtaining their first full licence and thus are subject to the New Drivers Act.  It is not a disqualification, and so you are entitled to apply for a new provisional licence straight away and as soon as you pass the theory and practical tests, your full entitlement will be reinstated.  Whilst there is no minimum period that you must wait before applying for a new licence, in practical terms, much depends upon the availability of test dates etc.