How would you test what constitutes a relationship?
I've lived with one housemate for 4 years (just us on our own), so twice as long as the Australian deemed relationship period. We're definitely not together!
Having children is no proof of a relationship either. All it proves is that you had sex at some point previously. It does not mean you are together now.
Having a child together and living together? On stronger ground here, but what time limits would you impose? What if the couple don't want to be legally tied? What if the relationship is in trouble or failing but then suddenly whoops it was our 2 year anniversary last week and in addition to having serious problems in our relationship we're now married.
What if there is abuse or violence in the relationship and you've just legally bound the victim to the abuser because they've lived together for two years? All that will do is make it harder and more traumatic for DV victims to detach themselves.
What if you go "on a break" and one person moves out? What if someone is in the military, for example, and is away for months and months each year?
And how would you define the starting point of the relationship which sets the clock on your two year limit? Your first date? First time you sleep together? Move in together? Does him staying over at yours 4 nights a week and you at his the other 3 constitute living together even though you both maintain seperate addresses?
There are as many variables as there are people.
It seems to me that in order for this system to work and people don't find themselves legally bound to friends / one night stands / someone they hate, you would need to get both parts of the couple to declare that they're in a relationship and wish it to be recognised legally... A bit like marriage...
What is needed is better education. Not defacto legal contracts.