I've just been going through a discussion like this with one of my friends. She has been dating her boyfriend for over 6 years, and he won't even discuss the possibility of living together, let alone someday getting engaged or married. He is still living with buddies and reliving the college days years after graduation. She actually just left him last week after a lot of debate and emotional discussions because their future plans and wishes seemed so different.
I had dated my DH for almost 3 years when we got engaged, and it was a little over 4 years by the time we got married. We were still in college when we started dating, so we waited for a couple years after graduation to get married.
I think there are plenty of couples that live together forever without getting married, but they have to be in agreement that that is what they want. In a situation like this couple, where one does really want to be married, it's usually not a good sign for the future. It sounds like they need to sit down together over a nice dinner and seriously discuss where they see themselves heading in the future. I'm not a fan of ultimatums, but they do need to make sure that they are on the same page about what they want out of life. Maybe the boyfriend doesn't realize how important it is to the girlfriend (which, the girlfriend needs to figure out how important marriage is to her - would she be willing to stay with him without getting married?), maybe he thinks there's no need to get married until they want to have kids, and it sounds harsh, but maybe he's still not sure if he wants to settle down with her.