Pretty much from the start. There are always things you can perfect outside your dojo. Depending what you're studying form, stance, movements, falls, katas and all basic moves can be practiced on your own. I like to practice a few things I've learned in class on the next day, so that it stays fresh in my mind. You're going to need a partner to test the effectiveness of a technique, but practicing the form outside the dojo will help you get that mind/body connexion faster.