After a year, you're wasting time
No reason to have a girlfriend more than a year. You either sh-t (get married -- I suggest DON'T do this unless you want a fat, naggy wife down the road) ... or get off the pot (break up) -- I suggest do this in your 20's so you can develop individually and date tons of women to find out what you like.
Really, if you dont' know if a gal is marriage worthy after six months, you're completely wasting your time. There's nothing more to know. You know.
Also, why in the world would anyone marry in their 20's is beyond me. Each person changes individually and you have locked away your single life. Wait until you're 30 minimum. See the world, volunteer, help others, get in shape, go to as many Major League ballparks or sporting events as you can, attend your favorite musical concerts ... trust me, women will put the kabosh on most of this stuff.
* Also, no reason to live with a girl before getting married, too. Ninety percent of my friends who lived with their now-wives before getting married are now getting divorced or having a lousy marriage. They did the test drive, bought it, and are still disappointed.
Hang on to bachelorhood as long as you can! Most American women aren't worth it.