"So how long should I wait before I contact my ex?" This can be a very tricky question as the answer varies from person to person.
As some people take longer to cool down then others but to be on a safer side do not jump calling back your ex immediately after the breakup as you might sound a bit desperate and even if you make the right decision to give them time alone it's still difficult to judge how long to wait when you're confused and emotional.
Note: Just by saying sorry will not do any magic! You might be experiencing a bad moment right now but it is worthwhile to consider why you want to get your ex back.
Do you really believe that you are made for each other, or are you just jealous that somebody else will get him/her and that's the main motive that you want to get her back? If you come to the conclusion that you are meant to be together then take the time to think about what are your mistakes so that you can change them before anything.
You should make use of the time to think about what you're going to say to your ex when you do call.
Calling is scary and one of the main problems is that you know you want you're ex back but a part of you is scared of contacting them in case they say no.
If you don't contact them then at least there is still hope that they'll say yes.
If you want to increase the chances of him/her saying yes then I'd recommend getting outside help from someone who's been there and knows what its like.
So to summarize, the twisted answer to the original question how long should I wait before I contact my ex is that you should wait a for sometime to contact your ex, but not wait even a day to start your indirect efforts at re-attracting your ex's attention.