No, no, I'm not asking you that. I was only imagining that's the question Mark Zuckerberg and his wife, then girlfriend, had to answer a lot of the time. They dated for over nine years, and for a lot of people in America, that is a loongg time, longer than some marriages. In Nigeria, where overly long dating/courtship/engagements are not really encouraged, I can imagine the question would be even more common to long term but unmarried couples.
When two people are exclusive with each other for a long time, and are open to others about their relationship, those around them begin to mark time for them. People assume the lady is wasting her time and chances with other men, plus her biological clock is ticking. Others think the man is using her, and will dump her when he finds who/what he really wants.
Many believe they are sleeping together, and for those who think that way, this devalues the worth of the woman in the man's eyes, after all, why buy the cow when you can get the milk for free. I certainly do not subscribe to this way of seeing things. Then again, is there some sense behind societies' disapproval of long dating/courtship/engagement periods?
According to those who have studied these things, two years is the optimal time to date and get to know each other. These researchers say this gives enough time for those in a relationship to really get a fix on each other's personalities and stuff like how people react under stress. However, I don't think time is the only factor, and may not even be a determinant, at all. A couple can date for 3 months and within that period, one loses his job, or develops a health problem, something traumatic that they're able to overcome together. This will certainly bond them closer.