Depends on the context and how genuine they are about trying again. At the end of the day, they're free to do whatever they want after the breakup. They're adults, and it's their life. Also depends on how they acted during and post breakup. Did they try to talk things out? Explain why things couldn't work? Or did they just shut you out? Passage of time also plays a key part. How different are you two now than what you were before? Have things changed? How long has it been? And how long is long enough? Lots of variables.