I couldn't give you a definitive answer on that one buddy. 6 months seems to be on the high side, but that's just a "feeling". If I were in your position, I would wait until I was in the region of 2 months away from my target and then apply. Once you apply online and get through screening, you could always give yourself an extra few weeks by choosing a delayed assessment date. Whatever you do, don't go to the assessment if you don't feel ready for it because they can tell when you are not, and they will only give you a second chance after 6 months.