I don't think there's a one-size-fits-all answer to the perfect timeline for mentioning previous relationships. The length of the relationship, when it occurred, and whether there were any children all are going to factor into that, for one thing.
I was married for 16 months when I was 20-21 y.o. There were no children, and the divorce was over 11 years ago, but generally the marriage still gets mentioned casually with some regularity because having occurred at such a young age it is something of a cornerstone of my adult life. On the other hand, i was briefly engaged to the man that I dated for 7 years after that, yet only rarely refer to him as my ex-fiancÚ because the engagement had such a small role in our overall relationship that I just don't think about it.
DBF has a child with his first wife, so that marriage was out in the open from day one. He and his second wife however separated after only four months, and do not have any children together. He does not talk about her, and one of the few times he has he referred to her as an ex-girlfriend. When I called him on it, his explanation was that she felt more like an ex-girlfriend than an ex-wife, and from the circumstances of their marriage, I can understand how that could be a truth not an excuse.
I did know about both marriages from almost the beginning, because I did run him through the public records. When we met, I did title searches as a large part of my job, and in the county where we lived any domestic incidences (and divorce records) were filed in the same office as real estate liens. Even though he did not present any red flags of being an abuser, I wanted to run his name through the system out of an abundance of caution. Both his divorces had been filed in that county, so they both showed up in the search.
All this is my long-winded way to say that I can see (from personal experience), how and why a previous engagement or marriage might not come up right away, or why someone might run a "public records" search on their date.