I put 3-5 days as that is pretty average but it does depend on the color of the horse. White horses seem to fuzz up faster so I like to do them as close to the show as possible, have even clipped at the show LOL! But dark colored horses or ones that lose color after clipping I like to go towards beginning of the week of show so that they have time for their color to come back.
For the first show of the season we would clip about a month before and then clip AGAIN a few days before the show. We discovered that after several clippings, their coat got better and better. Detail clipping on their face we would do the day before their showing.