From my experience, all dumpers have eventually come back...and it's taken as long as ten years (for the one situation I thought for sure would never end in my favor, but then again, by then it was too late). For the guy I ended up eventually marrying (now divorced), he broke up with me two months after we dated. It took another two months before we were back together in a mostly "undefined" relationship, and another two months after that before we both decided we were committed.
On my part, I have dumped someone once. That was my ex-husband. It took 1.5 years of hell for me to completely bail, even after I KNEW it was over. I think that when the decision is made abruptly, a dumper might be more likely to come back. I am a very reflective and methodical person; I didn't just wake up one day and decide I was leaving. This was the result of a long and difficult and complicated process, and the damage had built up over a number of years. I still love him for everything we have shared, and while you NEVER know what the future will bring, I highly doubt we would ever get back together in the long run (especially since I'd lost all sexual attraction to him after about two years of being together). Sometimes incompatibilities are too serious to entertain another shot.
On the other hand, depending on the circumstances surrounding a breakup, I think it's possible to heal and to resuscitate feelings that may never have disappeared but just went underground. In my case, as the "dumper," it's not a matter of coming to my senses and realizing I made a mistake. Sometimes a breakup isn't a mistake, but the result of a deliberate and carefully weighted decision.