How soon you apply for your Theory and Practical tests will depend upon the following factors:
Therefore, for example, if you prove to be a quick learner, and are able to have lessons frequently, then you should apply for your driving test as soon as you have passed the theory test (depending upon the wait for your test centre).
However, maybe it takes you a little longer to absorb things, (our experience is that those who take a little longer often tend to make better drivers at the end) and you can only have one lesson per week: then you and your instructor will need to make an estimate of when you will be ready for the driving test, and make the application for it accordingly.
The waiting time for a Theory Test is short, typically five to fifteen days. The wait for a Practical Test is longer and varies greatly from one area to the next.
In all cases, discuss the situation with your instructor before you apply for either of the tests.