In the heat of show season, I try to keep them pretty well clipped all the time. Then a couple days before the show finish them off so that at the show I can concentrate more on fitting than basic clipping.
If you are inexperienced, I would advise starting EARLY so that if you make a mistake it can grow out.
For the first show in the spring. I block out about a month early to acclimate new calves to the clipper and get some of that winter hair off. Then about 2 weeks before the show, do a thorough job, and the weekend before the show get them in show ring condition.
Kind of random, but that's how I do it and it seems to work out well :?
we clip them most of the way out about 2 weeks before a show. then do touch ups the week before and at the show. but the rest of the year i try to keep them blocked out so i still have my lines to work with and keep them cool.