I don't understand why you'd spend a long time cohabiting before marriage. Isn't the willingness to cohabitate imply that you intend to marry them, and are making a huge commitment since you are entangling each other in your lives? As far as I can see if you need such a waiting period to see if you want to get married after cohabitation, you should not have moved in so quickly. The only thing that appears to be the difference between marriage and cohabitation is the legal ramifications, and that should be subordinate, right?
I say 6 months is enough. If you know the person you know the person.