depends on the person. how do you feel? are you over your ex and past relationship and ready to fully invest yourself in a new relationship? are you ready to give that person a fair shot? are you ready to be loved? etc
i think a general rule is the longer the relationship the longer a person probably needs to wait before dating. but that's just a general rule
It depends on how long you were dating and how serious the relationship ha become. Were you married? Engaged? Living together? In love? In a sexual relationship? Had you talked about making the relationship permanent? Long distance relationship?
The purpose of waiting is to give you time to resolve the feelings about your ex so that the breakup doesn't have a major impact on your next relationship; i. e., avoid a rebound.
For some 1 week, days even, for others months. For example, women can move on almost instantly after long term relationship. There is a behavior called "attachment pattern", I recommend to read about it (applies 100% on my ex).
If it was serious then : For two weeks two weeks, for a month a two weeks for a few months half that time, for a year 3 month max. For more than a year 4 - 5 months depending on how long you dated them.
The same day.
You can't put a time stamp on it,
You need to heal...
That being said, at some point you have to tell yourself enough and you deserve to be happy and move on.
So take your time, no rush, but don't wait 10 years or something.