There is no dative method. Get married when you feel you're capable of being a wife, and he of being a husband.
There are no numerical answers to life changing decisions, sorry. It will have to be on your judgment. Talk to your minister.
Given the wording of the question, i think she means when you're with someone already. And most likely, if you're dating, the long term intention is marriage anyways.
As an answer, i say as a good standard a year minimum. If you're going to be preparing to spend your life with this person, you need to take time to really see who they are, and get past the 'still trying to impress you' stage.
Also, there is a chemical released in the brain the first 6 months of a relationship that causes all those lovey dovey feelings. This is why so many relationships end shortly after 6 months, the chemical wears off, and without it, they realize that this person isn't so great after all. So you want to have time to have that wear off and see if you really want the relationship to continue. So those are the reasons i suggest a year, minimum.