Louise, I think it really depends on the person. I was married for 14 years, we were together for almost 17. Mr. K and I split physically in April, but I made the decision in early March and that's the date it "happened" in my mind. And I am only in the past few weeks, nine months later, feeling pretty good more often than I am feeling bad. And even so, I am still having a lot of trouble sleeping -- I turn off the lights and have nothing but divorce thougths going through my mind. Also, it's not a straight line, it's a jagged one. You feel better for a while, and then you feel awful for a while, and then you feel better, and then you feel awful, and over time there is more "better" and less "awful."
That's my experience, anyway. It sucks but it does suck less as time goes on.