When can I stop practicing kihon and kata and just do kumite?
Why would you want to do that? Is only doing combat is your idea of Karate training. All Karate-do elements are interrelated, kihon training, high repetitions, will affect your Kata and Kumite positively, so will the other affect the rest too. There is no way to bypass any part of the training, without degrading some part of the rest. All are part of a complete training, there is no bypass, no easy way. I truely hope your not looking for one either.
