Hmmm, Gill, I have to disagree with you actually. I know that some people who renew their vows do so because their marriage has been through a very rocky patch and they feel so strongly when they come through the other side of that, it's about renewing the promise they originally made and reaffirming that promise. So, for me, it doesn't dilute the vows, it makes them stronger and more meaningful.
Gems, in my opinion there is no timescale. A renewal of vows, as I suggested above, should be about reaffirming your love and your promise to each other. I am sure there are a few here who know but many who don't. DH and I renewed our vows 7 months after we first got married. The reason being is that we'd arranged, planned and booked (and paid for) a big wedding in August 2004. Unfortunately, just a few months before, DH found out he was going to Iraq. For reasons which I won't go into completely (but mainly to do with DS's 'legitimacy' and legal issues), we decided to get married before he went to Iraq. We did this at Chippenham Register Office on 29th January 2004. That is the day we were married 'officially' - it was just DH, me, DS, my Dad and Step-Mum. It sounds really unromantic but it wasn't at all - it was a lovely day. We didn't tell anyone we were getting married, just went and did it, we were home by 3 o'clock and went to work and told everyone the next day
Because we had everything booked for August and we couldn't get married
again, we arranged to renew our vows at the August 'do'. All our family and friends were there, it was a really special day and I will never, ever forget it as long as I live. It was, without a doubt, the most wonderful day of my life. My FIL did a reading, we had some lovely 'vows' about our children and our life prior to the vow renewal. It was very, very special.
Just because I love to show off pictures, here's some of both of our weddings (see if you can guess which is which

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You probably can't tell but it snowed the day before and it was absolutely FREEZING!!!