A former co-worker gave me a formula: Take the amount of time you've been involved with someone, halve it, and that's how long it takes to emotionally recover from a failed romance.
In my experience, that's been completely false!
Example 1: A woman I was involved with briefly, at age 22 - off-and-on for two months - whom I considered my first and only adult love, yet who dumped me in a cruel and abusive fashion: This left me deeply upset for a year or so, and in spite of her enormous character flaws, I don't believe I fully lost my feelings for her until age 24 or 25...?
Example 2: A woman who asked me out at age 25, to whom I said yes because "Eh, why not?", dumped me after two months. I felt fine in a day.
Example 3: A woman my age in a class I taught took a liking to me and started flirting with me around this time last year. It made me feel stupidly happy and excited, even though I thought she was a little...childish. We had to restrain ourselves on account of the student-professor relationship, but with maybe a month left in the course, we disclosed our feelings for one another. She assured me she was very interested and would wait for the end of the course. At that point, she became distant (skipping class) and less communicative; she finally rejected me by e-mail, after I explained to her that all the uncertainty had caused me to suffer. Nine months later, I still think about her (I just woke up from a nightmare about her), in spite of my clearer perception of her unattractive features, in spite of my belief that a relationship ultimately wouldn't have worked.
That's pretty much the history of my love life, by the way. (I had a girlfriend in my early 'teens, too, but we're still on okay terms.)
No real patterns, other than that my co-worker's been consistently quite wrong! On the other hand, I'm not like most people, and, in addition to my fairly extreme lack of success, I'm deeply insecure and ambivalent about all things romantic.