It can accomplished quite quickly, but is entirely down to the individual and also the results can sometimes be very confusing and not make sense at all, let me explain.....If someone is practicing once per week and someone else is practicing three times per week, you would expect the person practicing three times per week to way more advanced after six months, but it depends completely on the attitude of the karateka whilst training, if someone is totally focused and applying the concept of Ikken Hissatsu with all their fast moves and train once per week, they will be way stronger and progress much faster than someone training three times per week, but simply going through the movements with no intent, so it's all down to the attitude of the karateka.
"The difference between a successful person and others is not a lack of strength, not a lack of knowledge, but rather a lack in will." - Vince Lombardi