How Do You Decide It's When It's Time To Propose Marriage?

You have been in a relationship for a while, exclusively seeing just each other. You get along well. You've met each other's families. So, what next? How do you decide when it's time to propose marriage? Some people continue for years in a "relationship" but never graduate to marriage. They may even cohabitate and have children together. But some couples come from broken families and fear that a marriage will not last. Some couples come from families in which there was never a marriage between the parents. How does a couple decide? What if one is ready but the other is unsure? Is that a time deadline one should consider? At what point should the subject be brought up? Should a woman ever propose to a man? Should the objective in a relationship always be to ultimately get married? How do you decide it's time to "tie the knot"