Piqla,
They normally start the courses four times a year: one in Jan/Feb, one April/May, one June/July, and one Oct/Nov. But the courses get booked up quite far in advance, so its not just a case on starting the next course after you pass, you have to wait until the next course with spaces.
As it is about a 6-9 month wait at the moment, and considering selection takes about 3 months, I'd recommend applying a good 9-12 months before you'd actually like to start.
If you want to do Aerodrome its looking even longer at the min. Far too many people want to do aero and NATS seem to have decided they don't have a very large need for aero controllers right now - the Jan course for aero has just been cancelled, and I believe the November course that should have just started was cancelled as well (someone please correct me if I'm wrong)... so aero's got quite a waiting list
.