We've all done the "rebound" hook up after a break up, but I personally think the rebound hook up right after a break up is a recipe for disaster.
Right after a break up, especially a bad one it can sometimes feel like it's a contest to see which half of the now broken up couple hooks up with a different person first. Besides this "contest" being childish and immature, it can often lead to hurt feelings and regret. Often after the break up, the parties go out and get drunk, and end up hooking up with some random person at the bar. The next day, in addition to be pretty hung over, you will probably regret that random hook up. For whatever reason, either you feel guilty, you feel like you might have lost the chance to get back with your ex, or even if you don't regret the hook up at all, it's still not the healthiest route for the road to healing.
There's also the fact that it's not entirely fair to the person you hooked up with. They might think they have a chance with you, and it can be very misleading. You are not ready to be starting a new relationship. If the person you hooked up with knows that you just got out relationship, and hooked up with you anyway, and especially if they hooked up with you because you just got out of a relationship, they probably don't respect you.
I personally think that you should wait at least three months before the "rebound" hook up. That way you're in a clear state of mind, and there's a better chance you won't regret it. You also have a better chance of not falling into the "rebound relationship" or being a cereal dater, just jumping from one relationship to another. You're giving yourself some time to heal from the old one, and get ready for the new one.
A rebound relationship is also not fair to your partner. You're probably not over your ex, and not only will your partner end up realizes that, which will probably do damage to you new relationship, but your new partner will also end up very hurt. You can never really, truly be there for you new partner, if you're not over your ex. Even if that person is who you are meant to be with, if one of you is not ready for it, you could lose out on it, if you move too soon. Especially when mentally, you're not in the place to be in a brand new relationship.
You need some time to yourself. Single time is good. It helps you rediscover yourself, and learn things about yourself. You realize how you've grown in your last relationship, and where else needs works. Sometimes this is very hard to figure out, with the distraction of a partner. Taking that time for yourself, those months, are healing.