After living together, when is it fair to inquire about marriage?

So I’m 31 living with my boyfriend of a year and a half. I know he wants kids but I feel like once we’ve been together for 2 years, coming up on December, I’d like to know what our plans are. When is it fair to ask when we would get married and if that’s what he sees for us? I won’t want to wait around after 2 years if he can’t give me an answer.
After living together, when is it fair to inquire about marriage?
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