You can enroll in an adult class, and there is a time requirement for the general dojo to test for a belt, for example, it takes about 20-40 hours to test for a white belt to a yellow belt, and depending on the dojo, the time requirement will be different. When you reach the hour requirement, you will be able to get the content of your corresponding belt and can take the test.
As your level increases, the difficulty of the content increases, and so does the number of hours required.
All together, it takes 1 year to get to red and black belt at the earliest. To get a black belt, you need to be very solid in your basic skills. If you are practicing to red and black belt in one year, it will take about 6-12 months to get to black one, and depending on your ability to suffer and the amount of time you are willing to put in, the time it takes to get to black one will be flexible. If you have been practicing for two years, your basic skills will be solid, and you will need about 4-6 months to prepare for the Black 1 exam. The Black One exam takes you through all the kicks of Taekwondo, Taekwondo Chapters 1 through 8, Goryeo, Board Breaking, and Stunts. So, the preparation process from red and black to black one will be longer.
I hope the answer is helpful! ~
I am a taekwondo instructor / martial arts set coach
Wuhan Longyi Taekwondo Hall (Guanggu Hall)
Beijing Lilong Taekwondo Hall (Shuangjing Hall)