The best rule of thumb is to wait at least three months to start dating again. Ideally? Wait six months. For some people, they don't feel ready to date again, but they do feel ready to have sex soon after a break-up. Though this urge can be powerful, it's best to say "no" to yourself as sex with someone too soon can leave you feeling even lonelier than you felt in the first place.
The number one comment I make to new singles is that they must mourn the loss of the past relationship, and the mourning process doesn't even START until you STOP trying to cover up the unhappy feelings - whether through sex with someone else, dating, or using drugs or alcohol to dull the loneliness.
I always tell my clients that this strategy hurts more in the beginning - to not act out on the feelings in any way, but instead to sit with them - but hurts much less in the end.
PLUS: Check out Dr. Seth's book, Dr. Seth's Love Prescription, about how to stop repeating the same negative patterns in your romantic relationships. (Click the book cover at the upper right to buy a copy today!)