I think it depends on the couple. I knew my husband 3 months when we married and we are still married 42 years later. My daughter went with her husband 18 months before getting married 12 years ago and they are still married.
But we laugh talking about how you really don't know someone until you marry them. Even after 18 months there were surprises.
So it totally depends on the two people, how open they are are, how they communicate. I will tell you this when you are in love, really in love, nothing will keep you from that person - you would walk thru fire for them. You just know.