The thing is, you don't know what trials life is going to throw at you, so imo it makes no difference whether you've been together a short or long time
I agree that there is no way to totally guarantee how your life with your partner with unfold.
But I disagree that therefore it 'makes no difference' how long you've known each other before you commit. The idea is to give yourself a reasonable length of time to see what this person is actually like -- not how they behave at the start of the relationship when they're all loved up and full of hormones, but how they actually treat you day-to-day, how well you can fit into each others lives, how you cope when one of you is ill/down/has something bad happen to you, etc.
It's a bit like Pascal's wager -- the possible outcomes are:
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You can think they're the one and marry them straight away. If you're right, yahtzee! You're married to your soulmate.
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You can think they're the one and wait until you've lived together for a year (or longer) before you get married. If you're right, yahtzee! You're married to your soulmate.
3)You can think they're the one and marry them straight away. If you're wrong, you then have to get divorced. Boo.
4)You can think they're the one and wait until you've lived together for a year (or longer) before you get married. If you find out you're wrong in that time, you can end the relationship.
There's no advantage to picking number 1 over number 2, but there is a massive advantage in picking number 4 over number 3. It all hinges on whether you're RIGHT about them being The One, and that's something you can't be sure of.
Give yourself the best possible chance of marrying the right person -- what's the rush?