When should you start GMing?
I'd just like to echo the sentiments I've seen thus far, and add that my first experience GMing was also my third session of gaming. The first two I was a player, but by session three I was put in charge. In the beginning, there were no DMs. Somebody started without ever having been a player before (because that's how the world works, someone has to do it first), and plenty of people still do. Just so long as you have fun, and the other people in the game have fun, experience is not necessary.
So, GM when you feel you're ready. Or GM when you don't feel you're ready. GM when you're bored, or when you're excited. GM when you've got an idea. GM when you've got no idea. GM when you want something that no one else is selling. GM when you think what you're selling is better. GM when you're tired of being a player. GM before you ever get the chance to be a player. GM all day, and half the night, and then GM on the reverse schedule just for kicks. GM whenever and wherever you want to, because there's really only one "should" as far as timing for GMing, and that's when you have players.
Not when there are players, mind you, but when you've got players in your game. That's when you should GM.
Hope this helps, and I look forward to you joining the GM club. ;)