I think it's impossible to have a set time because it all depends on the relationship and the people involved. Some relationships are over long before the break up happens, so you're pretty much ready to go as soon as the actual break up takes place. Other times, it takes time to process what happened, heal from it, learn the needed lessons and move on with no baggage. I think as a rule of thumb, if you're wanting to date because you feel lonely, or like noone will be interested in you again then it's too soon. But if you're just ready to get back out there and have some fun, and see what kind of people are out there waiting for you - then go ahead, who cares how long it's been!