Normally, someone new to Karate will attend one session per week in the beginning, as the student progresses this will obviously increase.
I personally recommend training one session per week for the first three to six months, after this, a minimum of two lessons per week is essential.
There is so much to learn and practice, if after six months you are only training one session per week, not only will your progress be slow, you will also struggle to keep up with others in your class and because you will struggle there is a good chance you will become disillusioned.