I would still say that your in the early stages of your relationship.
Also, you might want to make sure that the both of you would be mature enough to handle the aspects of marriage. Are you incredibly messy? Do you keep a checkbook? Do you have a fairly steady income? Does she? Do you insist on going out with guy friends 3 times a week? Does she go out with her girlfriends 3 times a week? Do you handle disagreements well, and maturly? Can you argue without insulting a lot? Does she? Would you get up nightly when she hears a bump? or if she's pregnant, has some weird craving or need? Do you share in duties?
will she stick up for you, and you for her? Will you do little favors for her years later? Are you a person that has to be asked 10 times before doing something? Do you guys save up money, or spend most to party? Can you handle a long stretch of no passion or sex? Does she or you have a temper over small things? How do you both handle frustrations naturally? Is she really super supportive when you're sick? Are you for her? Do you freak about having your own money, or do you let her borrow, and will she do the same for you? Can you afford an bigger apartment, condo, or house? Do you know which buttons to push to get her riled up, and does she with you? Will you avoid those in an argument?
These are just some of the questions that a married couple should ask themselves and their partners before getting married.
Can you handle changes? The both of you will change. Unfortunately, not always for the better. Sometimes, very much for the better. Can you handle those changes?
There are some people who have never met before they get married, and, surprisingly, it works really well. There are some who live together for years, and then they get married, and it falls apart. There are people who are in love on minute, filing for divorce the second, so I can't give you a real time frame. Only you and your girlfriend can do that.
Just realize that a couple of months is really not sufficient to tell, unless you're really "in tune" with understanding her nuances and quirks. Have you had your first argument or fight yet?
Just start saving money, and see where this goes. I think the average for asking somebody to marry you is about a year. Any more than that, it starts being more statistically likely that a marriage will fall. But, like I said, there's a lot of variables in that, and it depends on the couple.
I would wait. Consider the friendship as "worth only a few months" even if you've known her for years, because being your girl friend vs being your girlfriend, fiance, or wife is a lot different. Consider this relationship as very new, and wait till you're DATING at least 6 months to decide. Then start talking about everything with her... Your past, her past, dealing with methods of handling frustration... etc.