Most Helpful Guy
If you feel ready to go on a date with someone, then you are. Everyone has a different timeline, don't worry what works for others, just worry about what your own body is telling you. It will depend on how long you dated, how close you were, how things were at the end of the relationship, how well you compartmentalize emotions, how stable the rest of your life is, how emotionally mature you are... so many things, that nobody will be able to tell you with any certainty when will be the right time.
Thinking about your ex is normal, and that will always be the case, especially if you haven't been going out and dating other people. You will not have any large, significant interpersonal memories being made to push those other ones out of the way and back into your 'memories of the past' bin.
Most Helpful Girl
I think you can start dating when you are completely over your ex. And to the point where dating someone new would be for yourself and not to make him jealous.