Right after she drops $10k on me to show her appreciation.
That sort of selfish attitude will take you far in marriage. Nobody is forcing you to buy a ring. If a girl is, then I'd spend some time rethinking that decision.
I bought my wife a ring that cost me a lot of money at the time. Not because she expected or demanded it, but because it's what I wanted to do for her. If you do it for any other reason then you're doing it wrong.