It depends on what kind of skill level you are trying to achieve. Some people can learn basic cooking techniques in as little as a week. Some people take months to learn how to cook. Others may need to wait for years.
The person who is learning to cook can vary in the amount of time they need. An example: Someone who has never cooked before may need more time than someone who makes regular meals. Certain types of cooking require more skill than others. For instance, baking requires more knowledge than frying.
A specific technique will help you cook faster. Once you are proficient in that technique, you can move onto the next one. It doesn't matter how long it takes to master a particular technique. Keep practicing and enjoying the process.