A thousand years ago when I was running - and did a sorts of events from 5Ks to marathons and tri's, I kept my numbers (and medals) for 2 years... after a while it was just this big mess, a pile of paper so now its all gone. I have kept the numbers and medals for my first 10k, marathon and century. What I still collect and keep are patches, which I love to get. They all go on my "patch hat", an old cycling cap that now weighs about 10 pounds just full of patches and pins.