When do I start showing him the rings I like?

Be
I'm 21 and have been with my boyfriend since fall 2012. He's 26. 
He wants to propose this upcoming spring (probably March-June 2016) but I don't want to show him rings too close to Christmas, Valentine's Day, or my birthday (March). 
I'm kind of particular about the ring. I want it to be an Old European Cut moissanite and vintage style. 
When should I start talking about rings?
Edit: in December he told me to start sending rings! Proposal will be probably May-July this year. 

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