The question that can never truly be answered. Everyone's experiences are different though we share similar pitfalls in our relationships. The common answer has been "one month for every year you were together", however, is this always appropriate? There seem to be more reasonable responses, and ones that can cover just about every situation. For instance, it may seem to hurt more and for a longer period of time if you were the one that was dumped, but that is not to say that the dumper does not hurt, too.
In some cases, a break or break up may be necessary to learn and grow from the relationship and having to recall memories can keep the wound open. This is, though, a good time to learn about yourself and how you handle relationships and how you can make things work better down the road.
I have seen some very insightful remarks here that are very helpful and relevant:
"It's a good idea to ask this question in a larger context: one that encompasses your whole life. The trouble with break-ups is that they tend to remind us of anything else in our lives which is "broken" -- where life is stuck, dissatisfying, or stagnant. So our angst about these other areas gets "triggered" by the upset of the break-up, and it starts to look like the break-up was maybe a big mistake: everything becomes painted the same color of black....Love and relationships is a tough area by itself: there's a lot of entanglement between our ideas of love and our ideas of who we are at a deep level, so a breakup can really shake someone's idea of self, and resolving the "trauma" of a breakup involves seeing clearly who you are and what your future is about. So you can't really separate this issue from the other issues in life."
"I think when people have touched our lives, we will never really forget them. Just we are able to move on and take the good bits into our next relationship."
While I do believe that sometimes people break up because they are together at the wrong time (as I mentioned here), this isn't a hugely common situation. Most of the time, it's pretty simple and easy to see what keeps a relationship from working (for example). There are some interesting things out there to help people cope with the loss of a relationship:
How to get over missing an ex. I tend to like Mikko's insight and he has a lot of articles that he's written, some of which are linked back in this one.
WikiHow!. The most helpful and logical I've read. Step by step "to do's" to help heal your heart. "No relationship is ever a failure if you manage to learn something about yourself from having gone through it..."
There's often a lot to think about when a relationship ends, but that doesn't mean that every day or every thought has to be painful until the end of time.