After about a month and a half after moving in together, having been long distance dating for about 8 months before that and known each other (online, with both of us on dial-up) about two years. We eloped 3 months later.
I think it's less time that mattered for me and more that we'd covered and wrestled with certain topics together - we'd discussed having kids and how we saw that going (mostly worked well though we had fewer kids than we thought back then), our ideal careers and how we would manage those (this didn't come to pass at all, but our priority of family over careers that we did discuss did), philosophical beliefs, handling family over holidays and aspects, and generally worked out whether or not we saw the future together in the same way and could deal with problems and changes to our plans that came up together well. For some people, that takes a while to work out, others face problems together sooner and find those sorts of things out sooner.
I vaguely recall advice of waiting until you'd seen someone ill before marrying them, something about seeing who they are when the armour is down (also really angry as well, I think). About a month into living together, my now-spouse was hospitalized and yeah, that really crystalized a lot of the things he and I had been talking about, made it all very real so when he asked a couple weeks later in the middle of a conversation about what we were watching on TV (it was not a planned proposal, it just came up), while it took me a day to consider it (18 year old me didn't want to rush it), I could see it and that's what mostly mattered to me - even not being able to see the future, I knew we could make great choices together. I still joke with him now that he's my best choice.