Don't do it until you're absolutely ready! This is going to be your calling card. It's one of the biggest mistakes that beginning voice actors make - they either make a reel that's unprofessional that one of their friends helped them put together, or they spend an enormous amount of money and get a banging reel that sounds and looks professional, but the problem is that those are tiny snippets of characters that you might have to sustain for four to eight hours at a time at a recording session, or over the course of 26 episodes of a cartoon show. If you are not prepared to do that, if you don't have the proper training underneath you to sustain those characters, then you're misrepresenting yourself and can damage your reputation before you even get started.