Well I have a show on sunday and I started carb loading last Sunday!
It all depends on the physique and metabolism.
The weeks. months before a show you are striping BF away so you are (generally) limiting carbs (either by a percentage of what you eat, carb cycling or Keto). This causes the glycogen in the muscle to drastically reduce (or disappear) making you look flat.
The days before a show you want to increase the glycogen in the muslce to make the muscle look full. This can take some people a week, some a few days or some just hours. If your close to your weight limit you may need to do it quickly on the day of show after weigh in, so loads more sugars and fast carbs and in big quantities. If like me you take a long time to fill out, you can just add an extra 100g or so a day, then some sugary foods on day.
Water is also a big issue with carbing.