It isn't so much about time but about your life situation and maturity. Are both of you intellectually, mentally, emotionally mature enough to make a life time commitment of this magnitude. Take into consideration that from your teen through your mid 20's you are changing, growing and maturing more rapidly than any other time in your life. I can say without a doubt that the people you and your girl friend are right now will not be the same people in a year, two years or three years.
Secondly, are you financially independent enough to get married. You aren't living with your parents, won't have to rely on others for money, room, board or help?
Thirdly, what about school. Many people feel that if a kid gets married before they complete their education, it significantly decreases the chances of them finishing.
I suggest, since you are both so young, that you get "promised," to one another. Promised means that you are faithful and that you have an intent of asking her to marry you, when the time is right. Once the time is right, then you can propose and become engaged. An engagement has no time limit and neither does a promise.