We know it. You're working in the GNOME to impress girls saying them: "Hey, I'm a GNOME developer" :-), so you want to add yourself to the AUTHORS file as quickly as you can...
To know when you can add yourself only think :-), if you've only fixed some typos don't do it. If you've added a great bunch of (good) code, then go ahead.
(Very personal opinion:) I suggest you don't add yourself to the root AUTHORS file. I think you should wait for some of the core hackers recognize your work and add you. (i believe it would be very much more satisfying to know that the "real" hackers value your work, probably this belief is why I am not there yet... :-) )