I think it's 1 month but it's a rather pointless 'rule' and not something anyone should care about.
My engagement ring is lovely.
My wedding ring cost a fiver.
DH and I had gone to Vegas to get married. We thought jewellry would be cheaper there and would buy wedding rings when we got there.
We spent a miserable day trawling round a shopping mall with either unaffordable Cartier shops or tatty tourist shops and nothing in between.
I had a bridezilla meltdown (hot, tired, no ring) and started crying and being a brat.
DH calmly sat me down, got me a cold drink and then came back with a cheapy, $10 'silver' ring from a tatty shop.
He said "Catgirl, I love you and I want to marry you more than anything in the world. Marry me tomorrow with this, and when we get home I will take you to any ring shop and buy you any ring you want."
Reader, I married him.
The $10 ring is still my wedding ring and I didn't replace it with a 'proper' one because I realised the ring didn't matter and it will alway symbolise how much DH loves me and it means more to me than a platinum bit of bling ever could.