First off: I think it’s a loaded question that misses the crux of the relationship, and what relationships are about.
Splitting it into to main parts – How long?
Some couples know after a month. Some take a few years. All are valid.
So it’s not a time thing. Then what is it?
It’s:
Communicating and setting up boundaries.
Being vulnerable and discovering each other.
Healing trauma that may be evolve to be road blocks down the road.
Recognizing self, needs, goals, dreams as an individual and as a couple.
Getting on the same page when it comes to key values.
Some more but I think you are getting the idea.
What you don’t see is a TIME component. It’s not sitting there waiting for the relationship to happen. It’s conversations, texts, arguments, dates, dinners, memes, cuddles, meeting friends and family. The team is MAKING it happen. In your unique relationship, however long it takes, is the right amount of time. The worst is “Well, we’ve been together for 4 years, we might as well get married”. (Not a team, not coming from a place of desire and love)
When people are saying “I can’t believe they are getting married after only 6 months”, what they are saying is “I can’t believe that they did the majority of exploring and communicating in a short time.” Some couples can do it, and kudos to that.
When people say: “Oh what happens if he/she/they change in a year or five or ten”, again, another question that misses the point. People do grow and change over time. In fact, it’s a most likely outcome. Which is a nice segue into…
How do I know I’m with the “one”?
The one doesn’t exist in the wild like a shiny Pokémon.
You don’t find them on a shelf in the store.
The one is the person that you CHOOSE to spend your days with; you go through this wild ride call life and CHOOSE to do it with this person. This person adds to your life and makes it better, and you do the same for them.
“Know”?. You know on a day by day level. You truly don’t know in the future tense, because, nobody can predict the future, otherwise, tell me lotto numbers please. That’s why it’s important to live in the moment.
Faith and hope is the bridge that gets you into the future. (No, I don’t mean in the religious way, but why I say every relationship has a spiritual component). It gets you through the days/weeks/months that you have to spend apart. It gets you through the good times and the bad times. It gets you through richer and through poorer, in good health and poor health.
(If y’all choose polyamory that’s cool too! The number of people is less important than the fact that you CHOOSE them)