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The courtship process time starts when the man clearly reveals his interest in the lady and it ends when he proposes to her. That time should be no more than 6 months. After that, comes the wedding preparations, and I've seen brides take over one year to plan everything. I think that maybe you are combining the courtship and the wedding together?

I was referring to time interval between when they once meet and begin an exclusive relationship (or at least I hope it would be exclusive!), and the day they marry.  If they have known each other well for a long period of time, they should not need that long. 

It's not outlandish to think that a protracted engagement gives the partners --- especially the woman --- time to "keep an eye out" to see if there is someone else they "like better", particularly if one (or both) of the partners has the notion in mind that they've "settled for the best they're ever going to get".  Some women, especially, pride themselves on "not settling" and remaining single for a long period, even for life --- or at least that's what they say, there could be other problems they don't want to talk about (sex phobia, secretly lesbian but not practicing, asɛҳuąƖ, hermaphrodite who lives as a female, etc.).  In all fairness, some of those things would be no one's business, and if they choose to tell people "I've just never met anyone I wanted to marry", I don't think that's lying, so much as not saying everything that's on her mind.

I think it's quite "outlandish" for a man or a woman to "keep an eye out" once an engagement to marriage is announced.  

I don't.  Think about it --- these couples who stay "engaged" for a year, two years, three years, until one or the other, or both, finishes whatever goals they have in life (finishing college or professional school, building up their business, what have you).  That's a long time for people to start getting tired of one another, it gives them a lot of time to grow disgusted with, or bored with, each other.  In the meantime, other people just come along, in the normal course of living.  Someone comes along who strikes a certain chord with one of the engaged partners.  They get to thinking.  It starts working on them.  they reason "well, we're not married yet --- maybe getting married isn't such a good idea after all".  They start wanting out of the engagement.  Granted, it is not as bad as getting divorced.  It does happen.  And keep in mind that our society is hugely money-driven.  Let's say that the woman meets a man, someone she never knew before, who makes tons of money.  She likes him and he likes her.  She's starting to get a little tired of her fiance anyway.  It might not take much to push her over the edge --- "TIM-BER-R-R-R", as Audrey Hepburn said in Breakfast at Tiffany's.  Broken engagement and she takes up with this new Prince Charming.  It's far from an impossible scenario.