If you are asking how long a session should be, every person who meditates will do it for a different length of time. Some people need just a few minutes while others need an hour or more. When you first start to practice meditation, your sessions may be longer.
If you are asking how long do you have to make meditation a regular practice in order to see the benefits, this too will depend on the person. Most people will start to see some benefits of meditation after just a few sessions. Even after the first session, you may feel a little more calm and centered. The more you practice the more benefits you will see.