A friend's astute husband once said to me, "If it hasn't happened after two years, it probably won't."

We're talking proposals, of course. Kate Middleton would have left four times over, per this man-wisdom. But after an eight-year courtship, the recently engaged young lady is now a soon-to-be royal.

Eight years is a long time. It earned Middleton the nickname "Waity Katie." Granted, she and Prince William began dating in college, but it really begs the question: How long should wait for your guy to pop the question? And when do you walk away?

Thirty-six-year-old Bonni from Stamford, Conn., waited five years. "
He always told me we would get married from the beginning. We would talk about it all the time," she explains. "If I ever wanted to have a serious conversation about it, though, we would end up fighting." (Red flag #1.)

Dr. Lillian Glass, author of "Toxic Men", says avoiding the topic truly is a red flag. "If you have to walk on eggshells, that is a huge signal that the person may be toxic for you," she says. "If they get defensive, it means they have issues and can't communicate freely. If you bring up marriage they can say they are or they aren't ready for it, but if they give you a defensive attitude, that is a sign of future trouble."

While Bonni's boyfriend did get down on one knee after five years, there were warning signs he wasn't ready: "He planned a wonderful proposal, and I could tell he put effort into it. However, he told me after that he didn't want to discuss wedding planning for two weeks and just wanted to digest it," she says. (Red flag #2.)

You can imagine how shocked Bonni was when her fiance broke it off seven months later -- four months before the wedding. "We were arguing and I said, 'Do you even want to get married?' and he said, 'No,'" she explains. "I'm not sure he'd have ever broken it off if I hadn't asked this question, and I am glad he did. He told me that night that he didn't want to get married, didn't want to have kids (which he always told me he did), and everything I didn't like about him he'd never change."

Bonni says at first she was shocked, but after she moved out she realized she really wasn't in love with him, just the idea of him. "I think I was afraid to admit this to myself because I felt that I wasn't going to find anyone to be with," she adds.

Relationships expert and author of "How to Be the One," Roy Sheppard, says he's seen this situation before. "Getting married has to be a joint decision between two best friends who want the best for each other, who trust each other and are equally committed to the relationship," he says. "If your man isn't prepared to commit to marry you, he might not be satisfied that you are 'The One' for him. More and more guys are petrified that they are just part of somebody else's plan."

So, if you're tired of waiting around or you're worried something like Bonni's situation might happen to you, is an ultimatum the answer? In Sheppard's words: NO! "No matter how you spin it, or how desperately anyone tries to rationalize resorting to this type of behavior, it's worse than a horrible ploy," he notes. "Ultimatums are often based on fear, poor trust and a lack of ongoing open communication. Pinning your hopes on someone to fulfill your needs absolutely guarantees he'll be defensive."

You do, however have the right to know where you stand in a relationship -- and whether that proposal is ever coming. Sheppard suggests letting your guy know -- calmly and quietly -- that one day you want to be married and to start a family. "That's not an ultimatum, it's basic assertiveness," he says. "This is healthy." Sheppard says your partner has a duty to tell you if he doesn't want kids. "If you do and he doesn't, end the relationship," he adds. "But don't do it with the desperate hope that he'll change his mind and do what you want."

Good advice if we've ever heard it. How would you handle a commitment-shy guy?