How long into a relationship would you start thinking about marriage?

Personally, I have been with my girlfriend for over 3 years, but I feel like there is still much to learn about eachother and about living together, before I would ever consider thinking about marriage. I don't have any plans for proposing for at least another 2-3 years. But at the same time, I see people getting married 1-2 years into a relationship... How do you view this situation?

How long into a relationship would you start thinking about marriage?
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