The date is whenever you decide that the de-facto relationship began. Obviously you need to be able to substantiate that with relevant evidence.
With a married relationship it is easy, as you can just use engagement and wedding dates as formal milestones.
With de-facto it is much harder and different for everyone, especially when living in different countries.
As a general guide, you could take the date when you formalised your relationship in some way, for example by moving in together and setting up some sort of household. As evidence there could be joined leases, utility bills, combined financial commitments etc.
If possible, it may be a good idea to look into registering the relationship as well.
Nick van Voorst
Registered Migration Agent 0640648
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Please note that this posting is of a general nature only. It does not constitute legal or migration advice and may not apply to your particular circumstances.