Psychologists usually don't hand you a neat checklist with checkboxes - not because they don't know, but because recovery after a breakup isn't a linear process. Everyone has their own pace, and there's simply no universal marker like "cross this red line and you're ready."
But there are signs that specialists pay attention to. The main one is when you stop idealizing your ex and see the relationship realistically, with all its pros and cons. Second - the ability to talk about the breakup without sharp chest pain. Third - when you're happy with yourself again, focused on your own thing, and see the future not as a dark tunnel. And it matters that you're looking for a new relationship not because you're afraid of being alone, but because you want someone by your side - that's a huge difference.
Rushing right after a breakup often ends with you trying to fill an old hole in new relationships or repeating the same mistakes. So instead of naming a specific month or timeframe, psychologists advise you to listen to yourself: if you think about your ex rarely and without anger, if you like someone new for who they are and not because they're "not like..." - that's when you can try.