It is not the question of "how long" but "how ready are you to go into marriage?" Can you two stand by your own money-wise? Do you think that you could live with that person forever? Can you accept his/her flaws and annoying habits?

One could date someone for 8 years, but could never truly open up and get to know each other because he/she was just used to the other persons presence and not "love" the other person