This depends on how much time you are willing and ready to put into learning the clarinet. It is easy to learn the basics but to gain mastery in octave ranges, learning how to move to the next register, how to hold the mouth piece in place, where to place your fingers and how to read the musical notes can be a bit challenging but with daily practice you will get better.
If you have experience playing any of the wood wind instrument like the saxophone, learning the clarinet will be a lot easy. In conclusion, the amount of time you spend practicing will determine how long it will take to master the instrument.
If you practice instruments like the flute, trumpet, or saxophone every day for about a year, you will learn them. Learning clarinet is no different, and one of the biggest accomplishments and hurdles you will face is learning out to read the music you will play.
After that, it comes down to learning how to place your mouth (embouchure,) where to place your fingers, and learning what tones match what notes. Practicing every day is a must to truly learn and memorize not only the notes on the paper, but how to place your mouth and fingers.