For me it really depends on the circumstances of the break up.
1. If I broke up with her, it's extremely easy to move on. Chances are I was wanting to date someone else anyway which is probably why I broke up with her so I move on pretty quick.
2. If she broke up with me, then I stop talking to her altogether (because all the girls that have dumped me want nothing to do with me afterwards so I respect that). It can take anywhere from months to a little over a year for me to completely get over it.
I've dealt with someone dumping me in a couple of different ways to see how it helped me move on.
1) Hooked up/dated another girl a few days after - Didn't help. Plus on top of that I had new drama with these new females.
2) Stayed single for a year or more - Didn't help. Sat around all day, obsessing over what I did wrong, what I could've done better, what's she doing now blah blah blah.
3) Started dating a new girl no more or less than 2 months after - The best. I've given myself 2 months of singleness, got my head together, licked my wounds and ready to get back in the scene. I'm healthy and ready to give all my attention to this new woman, and the more I spend with her, the more the old one is forgotten.
Now the next thing is it depends on how long the relationship was and the quality of it which determines my reaction post break up.
If I've been in a relationship from 6 months to a year, I miss more of the ROUTINE of the relationship. Being used to sleeping next to her every night, having someone there to cuddle - just always having that reliable, stable, female romantic presence in my life. I'm more upset about the routine change, the empty hole that her absence causes.
I miss her too obviously, but most relationships I've been in that long, I begin to realize how we don't actually fit well at all. Things begin to come out that weren't issues in the beginning so I know in my mind it's going to end at some point, I just don't want to throw away the routine I've developed over the past few months so quick.
As for breaking up with someone after a short period of time, I don't have this same problem with routine issues. It kinda sucks but it's not a big deal to me. I more obsess over the idea of "what could have been" since the relationship wasn't given much time to mature.