I seriously doubt it would be easy to WIN a claim over assets earned during that time.
I know tons and tons of people that have been dating forever (Argentine guys don't seem to like to commit too easily..ha ha). Some of them have been with their girlfriends 7, 8, 9 years and some nasty break ups with the girl basically saying "sh*t or get off the pot" and the guy got off the pot so to speak and break up.
I've never ever heard of someone getting something in a common law type of situation in Argentina.