Okay, by six months I was already engaged to my wife of 20 years (and counting). HOWEVER! I had been through one marriage, she through two; together we had been through her father having major heart surgery that was 60/40 for him pulling through; we were both in our 40s and dammit, not getting any younger. Had we been 20-somethings? Yeah, six months would have been pushing it.
However, we didn't have our first real fight until after we moved into our first house together and yes, there are things that need to be aired out before final commitment (as we both should have learned from our first marriages!). In our case it was about money (major factor in her first marriage cracking up) and abandonment (me and my first marriage). In our cases, we also knew that big fights are not relationship-enders, but that was another older-and-experienced thing.