I totally agree with all of John's points, and can only add to them.
It does worry me when I hear people say that they "want to get married" or the like. Often these people aren't even dating anyone, they just like the idea of getting married, then when they do meet someone, then the draw of the idea of marriage is often stronger than the feelings to that other person.
Marriage is about another person, not an institution, although marriage is a gift from god, and so is the other person and indeed everyone we meet on this planet.
Consequently there can be no set time, other than the time needed to "check" if you like if the other person is appropriate for you, and indeed if you are ready for marriage, and everything that that entails. What are your values, core beliefs, spiritual and otherwise, desires, wishes, what are those of the other persons, and do they match, and if not does that matter and can there be compromise. Then there may need to be time for this to settle and proof or evidence to be obtained to backup any feelings.
As for "love", that is obviously important, and can happen at any time and grow at any pace throughout this process to a point you think is totality. This is not semantics, but notice the difference between "love" and being "in love", I love God, but I don't want to marry him (or her).
I have been married and divorced and can relate to the importance of what John has said above, and did a lot of self examination after I got divorced, including writing down on paper the answer to a number of questions, such as "what is love" (for me), what would it take for me to be "in love", and "how would I know", there was overlap in the answers.
Still, even after taking counsel, through prayer and talking to people, you will make the right decision at the right time for you and your new spouse.
Simon
UK