When should I tell my Ex-Girlfriend I'm dating?

You should just say it next time you talk to her... at first when hadn't read the entire question I was about to write why would you even tell her it's not her business any more who you date... but if you're friends then you should just tell her that you're not dating any more since she broke it off so it's obvious that you won't spend as much time together and talk on the phone as mush or whatever and that you're just moving on and dating other people. If she freaks out, then may be she shouldn't have broken it off with you, and you can tell her to make up her mind... she could stay with you and have you all to herself or she can have you as a friend and get used to the idea that you're gonna have other girlfriends... she can't have it both ways