I’d say a minimum of 18 months! (Consistant dating).
Even longer if you barely see one another! You want to see how they conduct themselves through ups and downs, happy, sad, angry, financially,resolve disputes with you when you disagree, as well as if they're romantically & sexually compatible with you. The first 6 months to a year is just the "infatuation phase" of most relationships. It's only after that time do we get a glimpse of what life might be like with them for the long haul.
Marriage should not be entered into lightly.
(Both people) should be on the same page and have their lives in order. It takes more time to get to truly know someone than most people are willing to invest.
Awhile back I wrote the following hub offering advice to men but it could as easily apply towards women.