At what point in a relationship would a witch or wizard tell a muggle partner about the magical world?

Would there be a rule or law? Would it be necessary to get ministry approval?

Would it be after a set period of time or after the couple got married? What about whirlwind marriages or people who don't want to get married and just live together for years?

What about after a break-up? Would the muggle partner's memories of the magical world be wiped, or the memories of the whole relationship? Would they be allowed to remember it all?

What about a couple with children - would the children's memories be wiped too, or just the memories of the partner?

I'm just wandering about the details as I've got an idea for kind of half a plot at the moment and the direction it goes is really determined by how this would work....

In my head, you would need ministry approval to tell a partner you weren't married to... And if you were to split up then only the memories of the magical world would be wiped. I'm just unsure how this would work if the muggle didn't consent to the memory charm or if there were children involved....