Limitations of the statute of secrecy?
So, for my latest Cotillion fic, Marlene is dating a Muggle and has been for a few years. They're in their twenties, and Marlene has been able to explain away going to Hogwarts as a scholarship from a private school to him, but now what I'm wondering is what the limitations were, if any, regarding telling your boyfriend/girlfriend about magic? Do you think there's some prerequisite time that you have to be with someone before you can tell them? Or do you think it might be marriage only?
The thing is, I know a lot of this will be speculation, as I don't think it was ever said exactly in the books but I am honestly unsure how the Ministry kept hold of the Statute of Secrecy considering how many wizards and witches married Muggles. So if anyone has any ideas or, possibly, in the chance I missed it in the books, a page reference/quotation about how wizards broke the truth to their Muggle spouses, I would really appreciate it :)