There really is no set timeframe. It varies from one man to the next. Honestly it's not about the "time" but "who" you are with is the most important factor. In this day and age there are many couples that moved in together NOT as a pre-marriage requirement but simply because it became more convenient than packing an over-night bag and driving across town.
One person was always over the other one's house and they were paying rent for two places...etc
In fact my guess is more than half of the couples living together moved in without having any "future marriage discussion".
If you are serious about getting married to someone the discussion of marriage should come up (before) moving in together.For example a couple dates for a year or so, a proposal is made, they move in together, a year later they get married.
I tell women all of the time that men do not go "looking for a wife". More often than not "we discover" we have found her in the woman we are with over time. For some guys they don't realize she's "the one" until after she moves out.
I would say by the time you have lived together for 2 years or less.That's plenty of time to see if you can live with their various habits for a life-time. :-)
One more note for any woman that is tired of waiting to be proposed to....It's 2009! There is nothing wrong with you proposing to your man. (You don't need to buy a ring) to ask.
If you are rejected or get a less than enthusiastic response then move on. Life is short!
Generally speaking men and women reject one another differently.
Men reject women by NOT asking them out. (Indirect rejection)
Women reject men by saying NO when asked out. (Direct rejection) If a man is not asking he's rejected the idea of being married to you.
If years are rolling by and a man is not asking his woman to marry him it's because he does not see her as being wife material nor does he want to risk going through the legality of making it "official" knowing deep down inside his view of your relationship is day by day, month by month, and not "forever".
Some couples live together longer than many marriages last. Examples: Goldie Hawn & Kurt Russell or Tim Robbins and Susan Sarandon. Whether you get married or live together there are no Guarantees in life. Nothing is written in stone.