Marriage, who, what, how, when????

I have been thinking about this issue so much lately. I think its the society's code of conduct that says that a woman who has finished college, has a steady boyfriend, should be thinking about marriage. And, boy, i have been thinking about it so much lately. The problem is that i have not found the answers to the questions i have. First, when should one marry? at what age? after what happenings or achievements? Second, who between the girl and the boy decides its time to get married.
Picture this, you've been in a relationship for three years, going to the fourth one. You love each other and have experimented on almost everything. Your friends say that you are a great couple and you can feel that you are.
Now the biggest question, Is it time? Is it the guy's responsibility to ask the girl for marriage? and what if the guy has no clue and the girl decides to ask? I really don't get it, what should be happening and what should happen? Am usually good with answers but these questions continue to puzzle my mind. Please answer me? I need to know!!!